1. This is possible. Not sure why the vampire needs to be insane, but it's do-able. The asylum would just be its hunting ground where it preys on inmates unable to defend themselves (locked in rooms which it gaseous forms into) or straitjackets and which no one believes (assuming it can hide its bite marks as self-inflicted wounds.
2. Entirely possible. Inmates are trapped, likely uncared for or with anyone to worry about them. If the experiments drive them insane well... time for more experiments.
3. This is unlikely or needs a different reason. Certainly they could have access to anyone who dies in their morgue, but it's just as easy to get corpses from a cemetery, graverobber, or just buy them off adventurers.
4. This is possible, but then you just need to figure out the assistant's motives.
5. This is useful if you want the asylum to not be the 'bad place'. Where the staff or administrator comes looking for help or exterminators. Possibly the staff member comes looking because the administrator or upper management don't want word getting out, thus requiring the PCs to go 'undercover' as staff or (more likely) inmates, by acting crazy enough to get committed. Then they investigate, possibly suspecting the administrator (who may or may not be up to something, even if not the killings or attacks).
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1. If a vampire, they'd need a coffin to sleep in. So they aren't likely an inmate, even if inmates never get any sunlight time. Likely they'd be either an insane predator stalking the halls, or they'd be running things from behind the scenes (though charm, domination, etc.) This would be an okay villain or enemy if you want one that could actually have exterior connections, contacts, and use minions (including charmed or dominated innocents). While the asylum grounds might be termed a business allowing hunting the halls or common rooms...individuals' rooms or cells might technically still require invitation, meaning they'd need to either be the de facto or technical lord or owner of the property or needing permission from the owner.
2. It doesn't have be an evil wizard. In fact, could be a neutral one with good intentions. That way detect evil spells don't work. In fact, you can leave detect good scrolls or spells around, which a wizard will be able to Spellcraft, and realize that if they just think good intentions of actions or deeds they will do, they will detect as good (but they should actually do these deeds afterwards, even if it's donating to the poor or charity or anything else).
You need a reason for their experiments. Creating life, curing insanity, returning a loved one to life (for some reason normal resurrection isn't an option), improving the human (humanoid) form mentally or physically, creating a perfect creature, making lots of money, fame amongst peers, etc. Then you can come up with a plan and methodology.
3. Again, a villain here would be more likely to be selling dead bodies out of the asylum rather than killing inmates themselves.
4. This probably works better if the a concerned staff member mentions problems to the PCs and gets them to investigate. They naturally suspect the administrator, possibly even getting him removed and the assistant takes charge thanks to their interference. More likely the assistant should be a side villain to the actual problem, ie. robbing inmates, selling medicine to the blackmarket, selling corpses to necromancers and doctors out of the morgue, etc.
5. Any suitable stealth prowler might work, but ones that don't leave obvious marks or marks that can't be explained as some other injury work best.
An Allip would fit thematically, possibly an inmate that died. Possibly the Scribbling Allip variant, to leave mad rambling scrawled across padded room walls. It might not be killing people (I don't even think it can really), but it should be 'juiced-up) with temporary hit points from feeding on inmates if the PCs encounter it. This is likely to be a secondary creature as well, since it's unlikely one or even two could really pose a threat to PCs other than knocking them out with Wisdom damage.
A Skincrawler ooze might be interesting, possibly a rash of them on various inmates (whose complaints of fatigue or disorientation are largely ignored).
