A vampire "can't" enter a residence without an invitation.
Forbiddance: The vampire can't enter a residence without an invitation from one of the occupants.
Source : Monster Manual, p. 297.
What happens if a vampire tries to enter a residence without a valid invitation? To be clear, I'm not asking for clarification on what a residence is, what an invitation must consist of, or exactly who qualifies as a resident eligible to issue an invitation, but about what happens when the vampire truly lacks a required invitation.
- Does this work as a mental block on the vampire, so they cannot even attempt to enter (e.g. their brain essentially "shuts down" or their body refuses to take action that would violate the prohibition)?
- Does the vampire encounter some sort of magical or physical barrier at the threshold of the residence (e.g. some sort of forcefield)?
- Can the vampire physically enter, but they experience some sort of punishment (e.g. taking damage, receiving instant death, being teleported away, being slapped with a fish, etc.) for having transgressed the rules?
In other words, is the prohibition more like a Star Wars-style droid restraining bolt (where the subject is literally being mind-controlled into being unable to perform the prohibited action), or is it more like a social skill where vampires know that while they can physically enter a residence without an invitation, it is just not something that vampires do and offenders are subject to social ostracism or being hunted down by the Fellowship of Vampires Code of Conduct Enforcement Patrol?
In response to a comment by Owen Reynolds, I'm not so much concerned about vampires being forcefully shoved into residences (as that case is covered in a linked question), but on what happens if the vampire proceeds to try to enter out of ignorance, stupidity, deceit, delusions of grandeur, etc. For example, if a PC tricks a vampire into believing they have permission to enter when they actually don't (e.g. "That is my house over there, you may enter." when in fact the house is owned by a hostile NPC and not actually occupied in any way by the PC), would the vampire instantly know that the permission is fake or would they find out when they attempt to step over the threshold? Would they successfully cross the threshold, but later experience a crisis of conscience when they find out later from a legitimate occupant that they unwittingly transgressed one of the foundational ethical rules of vampirehood?
Similarly, if I allow a PC to play a vampire and, one day, they see what they think is a guard tower but is actually the residence of a wizard hermit who has not in any way granted permission to the PC to enter, is there guidance on what I should tell the player or is it truly uncharted territory? E.g. do I say "For some reason you just know that entering that building is a bad idea and thinking about it fills you with dread. Is there anything else you would like to do here?", or would something like "You proudly approach the tower, but upon lifting your foot to cross the threshold, you are hit with a strong bolt of lightning which throws you five meters from the house. You take 2d6 lightning damage." be more appropriate?