iserith
Magic Wordsmith
I don't think it's all that slim. Strahd just waits until the PCs are sleeping and either wakes one up or approaches the one on watch, charms them, then leads them away from the others.
And knowing that, the players choose to use the buddy system when they rest. Or they die.
It's really all in their hands. You don't have to do anything except make sure they are clear on the risks.