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.