the preferred nomenclature is "come back with a warrant".

Don't forget that a door mat that says "welcome" counts as consent.

No. It is magic so they would not be able to enter partway through an answer as doing so would make it clear that the vampire knew it was really a no.

It seems to me that the wording itself is unimportant, but rather the intention. So I would imagine no

This inspired me to keep a handheld mirror near my front door, for when someone inevitably asks if they can come in, I can grab it and do a very obvious vampire check

The requirement isn't that vampires need to hear you say "You can come in," it's that you need to extend them a formal invitation and lower the barrier that protects your home. Theoretically, you could lie to the vampire, but they'd probably notice your barrier and wouldn't get fooled.

...you do have a barrier right? You properly consecrated the ground before moving, drew the sigils, and cleansed all existing evil spirits? 'Cuz otherwise, any old spooky creature will probably skip the pleasantries and just get you.

A lot of people here are telling you that the answer is 'no' because the vampires must respect your true intent or rely on trickery to get you to willfully invite them in.

But the real reason is 'no' because vampires aren't real.

No, vampires usually leave that sort of "exact words" trickery to faeries and genies.

Imagining a vampire showing up to Wayne and Garth's studio.

"You may come in...... NOT!"

I think it's safe to say that intent is what matters, not the technicality of communicating that intent. https://en.m.wikipedia.org/wiki/Intention

"You mayn't."

this is why you start the sentence with "no, you may not".

a lot of humans do that, too. cut you off early and pretend they didn't hear the second part, stuff like that. happened to me a lot. caused me to rearrange word-order a lot.

I would say, no, because the same magic rule prompting the vampire to ask permission in the first place also requires the answer to be complete. Otherwise, why bother? They would dart inside even before you had a chance to say "you" with the excuse that since you were taking too long you probably were okay with it.

I just realized that I'd be pretty safe from vampire infestations. I hate having visitors, and will make (up) any excuse to avoid them. "Sure, but I was about to leave to deal with a work-related emergency. I don't know when I'll be home."
...and then they can sit there alone until I see them leaving on my door camera.

I don't mind visiting others, because then I can leave when I'm spent. At home, however, it's where I expect to be left alone.

Depends, is this vampire known as Brock Turner?

They don't need to they just evict you instead.

A vampire mesmerizing a victim into allowing entry always felt like cheating to me

If you live alone and vampire shows up at your door with a gun and shoots you dead, could it then enter the house

Thats why you summon Abraham Lincoln, the Vampire Hunter

i guess not.

hollywood says vampires have to sparkly shine first.

some vamp lore says it is your invitation that counts and not the permission part.

some cultures need visitors to declare themselves human when knocking at your door.

They're fast, faster than you can imagine, don't look away and don't blink. Blink and you're dead.

Just say no.

"No."

I'm sure it varies by setting but my head canon is it's about intent. They don't need to be granted permission explicitly, they just need you to explicitly want them to enter.