If Alfred and the Professor could fend off the vampires as easily as making a cross shape out of objects, why didn't they do that as soon as they saw Sarah come down the stairs? In other works, the "they didn't think about it at that moment" might work, but Abronsius thinks about vampires so much, he should have done this much sooner.
If I'm remembering correctly, they didn't make the cross intentionally, it was just sort of a happy accident and the professor is kind of scatter brained, so the "they didn't think of it at the moment" excuse still holds up.
Stuttgart production: The cross is made and Krolock screams "Bring her back!". Then we see the animation of the complete castle turning into dust. Which looks awesome, but makes no sense. So Vampires can't walk through crosses but as the Professor had one for some time in the near of the Count, they don't seem to be deadly. So why exactly is everyone dead and even the castle turns to dust? Eh?
As this troper recalls, the staging decision in question (i.e., producing a cross reduces the castle to ruins) dates back to the early writing process for the American version pre-Michael Crawford. It was apparently felt that the vampires' reaction to Alfred and Abronsius' crucifix wasn't dramatic enough to give the twist ending more impact (i.e., if all the vampires are dead, then it seems like Alfred and Abronsius have actually won the battle, which would make Sarah's surprise snack at the end of the show more shocking, to some anyway). Early drafts of the American version featured total destruction of the castle once they flashed the cross, including an eruption of fire, walls crumbling, and a river of blood pouring down the middle of the ballroom. This of course was not reflected in the final American production, replaced with an overly dramatic death scene for Michael Crawford. This troper's guess is that at some point, European producers also decided the events in the castle ballroom came off in this manner, hence the castle's self-destruction and a bunch of dead vampires. Evidently they repented their sin, for it has not featured in many other productions that this troper knows of.