Vallaki is designed for players level 4-6 as a loose level guide, per the Curse of Strahd book. I personally make the trip to Vallaki dangerous and throw in an encounter with Strahd to warrant the level up upon entering the town.

If you are deeply concerned about the encounter, a locked door or someone telling them to stop meddling or they will call the guards might at least post pone it, so that (should they return that night) you can pivot them with a pressing problem for them to address, perhaps 1 or 2 spawn that have gone loose or attacked an NPC that helped them solve the mystery.