So in my RPG campaign, my players are sneaking into a keep occupied by vampires. They were told of a secret tunnel that leads from the cliffside and into the keep itself. This tunnel would've originally been an escape route for the noble family that used to live there but has since been abandoned.
Why would it be abandoned? Preferably something monstrous or dangerous (for the heroes), but this tunnel would have been in use for hundreds of years until less than 2 decades ago.
The entrance is secret, meaning that not everybody is aware of its existence. Maybe it was known only to the head of the family and to the highest in rank butler.
The few to know happen to be dead or killed, for reasons completely unrelated to the entrance, before they can transfer the secret to their successor, and therefore the awareness of its existance was lost.
The most reasonable explanation is that the tunnel was in use for hundreds of years while the noble family lived there, then was abandoned when they abondoned the castle. When the vampires moved in, they simply didn't know that it existed.
Alternately, while the castle was completely abandoned, some monstrous thing moved into the tunnels, and the vampires then later moved into the main castle. The vampires decided they would rather coexist with the thing in an uneasy truce rather than suffer losses trying to eliminate it.
What the thing is depends on your campaign setting. Beholder? Awful giant slime? Unhinged wight? Really depends on what is available in your campaign.
The simplest explanation is that the original entrance to the tunnel from the keep is disguised for a variety of reasons. Assuming that vampires have memories that work much like our own, unless a vampire is actively using that secret entrance or practicing rapid egress from the keep, they're going to forget about it. Don't use the secret escape route for long enough and it will pass completely out of memory.
We see this kind of behavior in humans all the time with phrases like: "Hey, I forgot all about this jacket!", "I haven't seen this box of stuff since I was a kid!".
Perhaps two decades ago, someone redecorated the keep-side entrance to make it much harder to use. If the immediate threat to the keep occupants was low enough at the time, they wouldn't have been motivated to re-redecorate the entrance to make it functional again.
Secret entrances are inconvenient. For day to day activities there will be an easier route to use. Once the coolness of the secret entrance wears off people will use whatever is most convenient. Since this entrance is designed to be hidden and it wouldn't be much of a secret if everyone knew about it, all it takes is a small number of people to forget about the path they never use for it to become abandoned.
Perhaps it was built when the family were Catholic under a Protestant monarch (or vice versa). A fairly common scenario at various points in British history. The entrance then became an entrance for the kids when they were playing but due to lack of maintenance or an accident they were banned from using it and the secret was never passed to the next generation. The secret could also easily have been lost when the house changed ownership.
Perhaps it was a servants entrance (the family only used it for illicit affairs so it wasn't much talked about above stairs) and with the loss of the domestic service culture after the war it simply fell into disuse.
Surfaces degrade, walls and stairs become unsafe, anything near a river would flood.
Notes on lost secrets:
When we were doing works on my grandfather's house a few years ago we discovered he had a priest hole.
A friend's family bought a big old house and having lived there for a few years opened a disguised door that apparently up until then they hadn't opened, as they discovered an entire extra (servants accommodation) floor.
Collapse, illusion or not, made it look like it was blocked. Or something along those lines that make the vampires think the route is blocked. Maybe a landslide at the end.
That is probably a better option. Don't break the tunnel, break the entrance. Perhaps a landslide let it exit on a cliffside. Unless you can fly a death sentence. So then the tunnel stops being used.
But recently, perhaps another landslide, create a new route to the exit. This is what your heroes will use.
No one left
You said that the noble family who used to live there used the entrance. If the family fled, no one would know about the entrance, but townsfolk might surmise that, since no bodies were ever found or buried, there was a secret escape route that was used.
Alternatively, the entrance may have been used to bring in provisions and goods out of sight of the public. Without anyone to inhabit the castle, knowledge of the service entrance would still exist, but no one would use it if nobody lives there.
Too dangerous to be used as an entrance
You also said "cliffside exit". Depending on the height of the cliff and where the exit is, getting out might be easier than getting in. Maybe the exit once led to the bank of a river that has since been dammed, causing the water level to lower. Curiosity isn't necessarily enough to risk one's life climbing a cliff for what might turn out to be a cave, or worse - an animal den.
New inhabitants
Perhaps a dangerous animal has taken up residence in the tunnel and, after enough people didn't come back from "exploring the old escape tunnel at the keep", people stopped bothering to go there.
Vampires don't need to escape
If I knew the local keep was inhabited by vampires, even with a secret entrance they might not be aware of, I'm not going in there as an average townsperson. The vampires won't need to use the old escape route if they're not being presented with any serious threat. Disuse tends to lead to disrepair, and they may have forgotten about it, or never knew, if they took up residence recently from another place. I doubt they asked the townsfolk much about the castle before moving in.
You wanted something dangerous.
Dangerous for your heroes (no vampire I presume?) or for the vampires?
For the vampires a simple crack in the wall of the tunnel that was running along the cliffside would let light fall in during daytime.
For your heroes: Flooding. The tunnel is flooded and maybe under the floor there's a running river with strong current. Some parts of the floor easily break, letting the poor person walking over it fall into an underground river being dragged into a tunnel without air. The result would be drowning obviously.
This might also be manually made by the vampires, as the river could lead into a deeper cavern. Since they are undead and don't breathe, they could use this river as a shortcut or something. As the world you are in is medieval the heroes have no means of diving with gear.
Edit: I just though of another dangerous thing for heroes. The tunnel has gas oozing in from below. The gas fills some parts of the tunnel. It does not smell but is not breathable. So once the Heroes enter they get sleepy and then fall over and suffocate.
Vampires, again don't breathe. And for humans of that time it might look "cursed".
Changes to the water table.
Changes in irrigation and draining of marshes led to the the tunnel gradually becoming flooded decades ago.
The drought this spring means it's passable for the first time in twenty years.
(flooding due to nearby marsh drainage allegedly happened to a secret passage in the 13th century church building I used to worship in, used to escape to a priest hole in the house over the road - St Cuthbert's church and The Black Swan Inn, York)
Security.
They suspected, or found out, that word had got out about their secret entrance. And they feared it would be used against them. So they ordered it completely ruined.
The person set up to do the deed held a big grudge against them. (option: close relative previously bled dry). At huge personal risk, that person managed to not fully disable the entrance, and hide the vulnerability instead.
Then held that information close to the heart for two decades, waiting for just the right occasion...
