You are describing a classic predator-prey system
To justify how this works, you need to look at how predators and prey coexist in natural ecosystems. Prey species show a number of adaptations to reducing predation risk, including forming groups, engaging in vigilance and defensive behaviours, and "hiding" (whether underground, up trees, or concealed by vegetation. Forms of predator defence work because they increase the "hunting costs" of a predator - by making the predator incur costs - whether these are time or energy costs, or risks of injury - they make each hunt less profitable and so decrease the per capita risk to the prey.
So examining your scenario - humans live largely in towns, while the monsters live in the wilds. So your humans are already forming groups - this might just be a "selfish herd" (Hamilton, 1971) benefit (reduced chance of being the one killed with each monster attack), but will likely allow more organised defence. They live in structures which increase the search time and energy costs of hunting vampires. It might be that your humans take this further - narrow twisting streets might be harder for a vampire to fly down, particularly if the humans put up a netting and canopies (or even hang laundry across the streets). Sealed doors and windows will slow down a hunting vampire. Perhaps the humans also live in small rooms joined together by doors and narrow corridors, each with more doors. Maybe they even move between residences within the town, so that e.g. ⅓ of all residences are vacant at any one time - all to increase the search costs of the vampires.
From Optimal Foraging Theory, we can predict that the vampires will attempt to optimise their "energy" returns (in this case gaining sufficient blood to tide them over to the next meal), and so the distance they have to travel to the town to hunt, and the time taken to look for a human to feed on, and whatever energy they use up in the process, have to set against how much they gain from one or more victims. Even if your vampires don't have an energy constraint, they definitely have a time constraint - they have to be back in their shelters before dawn, so the humans just have to hold out that long (and for each individual human, it's about them surviving the night, even if that means a neighbour is taken)
[wouldn't normally pick wikipedia as a source, but the page on optimal foraging theory is pretty good: https://en.wikipedia.org/wiki/Optimal_foraging_theory]
Now to your question: why don't the humans leave?
First, where would they go? If they leave the relatively safety of the town(s), they'll be in the wilderness, where all the vampires are. Even if they can travel safely by day, at night they will be hugely vulnerable - in a much smaller group, without the protection of buildings and much closer to the vampires' roosts - any groups trying to leave face being wiped out. These humans may have little knowledge of where else they might go - what reason would they have for assuming that the wider world was any different to their immediate surroundings?
Non-human animals in the real world suffer high levels of predation without abandoning areas - which may be because they know where the food is locally (this would also apply to your humans), and the surrounding areas may have hostile groups of the same species - added to which leaving groups sacrifices that protection. In your scenario, it even even be the case that if local food supply can only support each town, humans might be hostile to humans from other towns - because they can't afford to feed extra mouths - or they might also be welcoming, as there's greater safety in numbers. (It's worth noting that if you're going for realism, towns only survive because of the countryside that surrounds them and provides them with food, so you might have to hand-wave away where the humans get their food. Maybe the towns are build above caves that provide substantial mushroom crops, and incidentally provide hideaways from the vampires.)
Real world prey species typically breed faster than predators - the above analysis will collapse if every person fed on by a vampire becomes a vampire - the humans will be wiped out very quickly (timescale will depend on initial number of vampires and number of "kills" per night). If however, vampires breed very slowly (which would be typical of long-lived species - perhaps vampire "reproduction" is different from vampire "feeding") then your human/vampire system will be an even better match for real-world predator-prey systems.
You'll also need a low density of vampires, particularly if humans are their only prey.
So finally, to get to your original question: How is it possible to rule over the whole country?
Humans are scared, and subjugated. They survive as best they can. Each individual has a pretty good chance of surviving each night's vampire hunt (maybe vampires only hunt few times per year?), such that they are likely to survive long enough to have children and raise them to an age where they can reproduce. Living in towns provides that survival edge, but also creates an effective prison. It is too dangerous for the humans to do anything else.
Of course, this raises the question of what sense Dracula rules over the country? He presumably doesn't collect taxes (what would he need wealth for?), and doesn't need the humans to act as an army to attack other countries (or defend against their aggression), as the vampires would be a far more effective force. It would seem that he doesn't so much rule a country as manage a hunting territory.
The other literature you might want to look at concerns the concept of landscapes of fear e.g. https://www.cell.com/trends/ecology-evolution/fulltext/S0169-5347(19)30019-9