The earliest monsters that can turn invisible are orc wizards and if you can corner these in a corridor where you know where they are most chars can just bash away at them and win without seeing them.
For vampires this generally doesn’t work. If you have some item with see invisible (or some divine power to show them) vampires are mostly harmless by the time you encounter them. But if you don’t, they have enough evasion to avoid most of your hits when you can’t see them and their vampiric draining helps them to counter the occasional hit that goes through anyway. Additionally they can confuse you but even if you have enough willpower to mostly ignore that you can’t just melee them. Good concept game wise, you need to figure out a way to get around the invisibility.
But what are you supposed to actually do? A scroll of fog or poison will show you where they are but that doesn’t help compared to just being next to each other in a corridor. A scroll of revelation will make them visible for one turn which is not enough to fight them. Running away is always an option but is that the only save play available until you either see invisible or are so much more powerful that you can just ignore them?