So it's the vampires' half of the initiative, right?
Vampire A comes down as a bat, grapples the party's wizard from behind. Pulls the wizard away hostage style like "move and he's dead".
So it's like, front to back:
Vampire B moves with them, taking an overwatch action, like "anybody move and you're toast."
Party's thief has an invisibility ring, sneaks behind Vampire A, garottes him from behind, not killing him but preventing him from attacking. So:
Of course then bodyguarding Vampire B takes his snap action to grab the thief from behind. So it's like:
But then it's still the players' side of the initiative, so then the party's fighter runs up behind Vampire B and garottes him. So they're all in a horrible deadly conga line...
And it's time to roll initiative again...