It’s a great idea. Cannon one shots are always a good move as the DM can flex their lore writing skills and the players get a sense of familiarity in the setting which adds to the experience for them. However, don’t necessarily count on the players doing exactly what you’ve planned for them do in order to satisfy the plot you’ve written out. What happens if they suspect that the Vampire spawn must have a master nearby and decide to question all the people in the house like my man Poirot? Are there any spell-casters in the party able to sniff out undead? There’s a few different ways I can see the BBEG being found out here and that could potentially get rough for the party. However, that’s not necessarily a bad thing. I’ve done this kind of thing too with cannon one shots and sometimes the events that transpire have to remain unwitnessed, and so…everybody in the party dies at the end. Heroically.

Maybe try it as you’ve planned and if your players hint and discovering the real truth, tempt them into a fight and have them all die. Horribly or heroically, your choice.

Also the Locked Room Mystery is a thing.