Suggestions for building an electricity vampire

[Aergoth]

My suggestions are as follows:

Given how Flesh Golems work have it drain initiative and inflict a slow effect. After it's drained so much, give it an effect like haste, and have it play a little like a quickling, zipping around with spring attacks and the like.

If your vampire has gaseous form, add an effect something like shock shield to it, where creatures passing through the cloud take an amount of electricity damage.

And tie some of the nicer abilities (domination for example) to its feeding, so it needs a full charge and then spends that charge to use them. This will give your creature a reason to keep engaging with the adventurers, or a reason to run away.

For a harder encounter, have the party fight it as it's just finished feeding and is at full strength. For an easier encounter, make it have to feed on the party and have them forewarned.