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.
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.