I can see where you guys are coming from, and part of me wants to roll with that. It's just another part of me still isn't sure it's fair.
By the way, I think I have already telegraphed it once in the campaign. The party was staying in the Kreszkovs' house for the night, after fighting wights that had risen to stop them from recovering the sunsword in the abbey garden. The players stated that they'd set a watch, and I randomly determined which one was on watch when Strahd knocked on the window. The player had his PC go and open the window, without bothering to wake any of his companions, and Strahd successfully charmed him.
Strahd asked the PC to let him in, but the player was clever enough to say that since it wasn't his PC's house, he didn't think he could let him in, so instead Strahd asked him to climb out the window to him. The PC's hit points were already quite low from the fight with the wights, so he dropped to 0 after only one suck from Strahd ... but I didn't have Strahd kill him. Instead, the other PCs woke in the morning to find the window open and their companion lying on the ground outside, cold and pale but still alive. Oh, and the sunsword was gone.
I am curious to see if, the next time Strahd knocks at their window, the PC on watch has the presence of mind to wake up the others first. If not, then OK, I guess they were stupid and deserve to get sucked dry. Strahd's definitely going to kill a PC next time he gets a chance. They let Ireena escape through the pool in Krezk, and they keep refusing his invitations to the castle, so he's starting to get fed up with them.
EDIT: As an aside, I note that Chris Perkins has said (on Twitter) that a vampire's charm is strong enough to force a PC to fight the other PCs. (I got the same response from Jeremy Crawford about a cambion's charm as well a while back.)