Is vampirism still worth using on a magicka DK?

I wouldn’t pick it just because older builds used it. Check the current descriptions for its skills and passives, then compare their benefits and drawbacks with what you want your Dragonknight to do. Those details can change between updates, so older build advice may not match your game version.

If the effects fit your playstyle, try planning around them. If they don’t solve a specific need, there’s no reason to make vampirism part of the build by default.