How long until you start questioning if it will work and invite doubt? Or will you assume defeat if the vampire fills the gap with, “why, thank you my good sir”?
My understanding of the idea with many interpretations of magic is they are all just ways of focusing your will on the world.
Ergo, the words aren’t themselves the source of power, your expectation that the words will result in a certain outcome is.
Therefore, if your intention is to deny entry is strong, there could be a fairly good gap.
But on the other hand, playing around to try and see could create doubt and uncertainty, weakening the effect.
If the vampire knows it is intended to be a no then it doesn't matter. The person could never finish it as long as the vampire knows the real answer is no despite the words being stated not matching up.
If they are magically forbidden to enter without permission, but also don't know every language or phrasing of 'come on in', then there is a magical way to know intent without needing to hear all the words.
Otherwise they wouldn't be able to work with nods and hand motions from people who cannot speak, shrugs and grunts from drunk college students, etc.
Hmm, this was debated already and in this comment they provided a peer reviewed journal article about intent vs language and understanding. It sums things up better than I could. They clearly cared a lot more than me about this.