Vampire, especially at stage 4 offers almost no bonuses, while I think it should add the most advantages to your curse. I thought about it and come up with some simple changes, that will make it worth while and not making it OP.
First things first is health regen. Vampires are supposed to be hard to kill, yes we have damage reduction based on our missing health, but health regen is a factor to survivability also. So in old fashion of vampire lore both in real world fiction and TES lore, make a static health regen bonus at night, it would fit the lore and vampires would be happy when its night outside. But, in PvP it can cause certain issues, so make that gear sets don't affect your health regen, you have what you have with vampire buff and that's it, but make it strong, as it's only night time buff.
Second is increased cost on non vampire abilities. Keep it, let it stay the way it is now, but, add a mechanic that the less resource you have the more damage you do, kind of like how damage reduction works. I think it would fit with current lore of vampires also. They get exhausted, they get angry, and dish out more damage.
And one of my favorite changes is to Arterial Burst. Make it ranged, it costs magicka, but it's melee, I don't know about you but I haven't seen much melee casters. Also to not make vampire magicka only, add stamina morph with the same mechanic, melee of course.
Blood Frenzy and it's morphs is another thing, it costs too much to use that ability, reduce the health cost a lot on this one, but remove health regen completely as stated in suggestion above while in Blood Frenzy. I saw vampires both dungeons and PvP and no one uses that skill. I don't have much ideas about it, but something has to be done about it for sure, same as Vampire Drain.
Vampires in rework are interesting on the paper, but in practice they need more work, vampire stage 4 offers almost on bonuses when it should be the strongest. Most cool but useless abilities are not used because they suck, the only skill that is worth while is mist form, and then again, only one morph. It seems to me like they haven't even tested anything while making changes to vampire.
Fun bonus: add reduced fall damage