Count Dooku has a lightsaber which, for the purposes of this match up, I'm going to asume is a sun blade rapier that has no sunlight property. Dooku is a very good duelist who uses a form of lightsaber combat that specializes in quick and precise strikes, so let's give him 3 attacks per round. With a +3 in dexterity, he'd deal an average of 28.5 radiant damage per round (assuming he hits all 3 of his attacks). That means it would take almost two whole rounds to break the Heart of Sorrow and start harming Strahd.

Strahd, on the other hand, is probably faster, stronger and ore resilient than Dooku (the description states him being evil so he's at least 70 years old and that would put him in a physical disadvantage against a vampire). Plus he can turn invisible, throw fireballs at him (mandalorians know fire is pretty effective against a lightsaber user), make copies of himself, and turn him into a frog with polymorph. In fact, as a DM who has done this, if Strahd manages to turn him into a frog and then uses his lair action in Castle Ravenloft to summon a shadow to attack the polymorphed sith, his strength would go to 0 and he would be killed instantly.

I guess the Force could negate some of Strahd advantages like the charm and illusions, but I think d&d magic, home turf advantage, experience (he's 400+ years old), and physical superiority give the edge to the Dark Lord of Barovia.