This is a comprehensive post about Vampirism and how I think it could be better.
The point of this post isn't to buff or nerf vampires in PvE or PvP and that balancing should be done at either end if necessary.

This is how Vampirism can be better not only from a perspective of lore but also player interactivity and be non-linearly rewarding.

Let's start off with the skills. These are the skills that make the vampire line try and feel vampire-like. I will also include the ongoing passives from the stages 1 through 4 of hunger.

[Bat Swarm] Active: Summon a bat swarm which deals magic damage to nearby enemies ever 1 second for 5 seconds.
-Skyrim Vampire lords have an akin blink ability which transforms you into bats. Really silly and awkward but a strong ability. It kites the edge of making sense lore-wise and this ability may actually need a nerf. This ability will not be discussed further on this topic.

[Drain Essence] Active, Channeled: Stun target for 3 seconds and do magic damage every 1 sec. Player recovers 150% of damage dealt as Health and Stamina every 1 second. Cannot be applied to targets that have recently been drained or fed upon.
-Great ability, bread and butter of the skill line and an occasional dependency when fighting against enemies that deal fire damage. Very buggy ability. Targets that are immune to being stunned are immune to the activation meaning they can't take just the damage portion or heal the player. The only notice you get is a little text in the top right corner of the screen that says 'Target Immune' and you can very easily die if you depend on the healing from this ability especially when fighting a boss or regular enemy that deals fire. Do not assume you can use this ability in a boss / elite fight or you will be disappointed most of the time. What makes it worse is that it will work on some elites and some bosses. Even random non elite enemies will be immune and the classification feels totally random.

[Mist Form] Active, Channeled: Take 75% Reduced damage from all sources. Invulnerable to healing magic and control effects.
-Underestimated ability. Only really useful for tanking extreme burst damage or for disengaging and running. Correct use of this ability can win scenario's that would be previously unwinnable.

Now that we have covered the actives, you would think that a lot of those sound really strong. Yes they are, in some cases too strong and a little pullback could be welcome but now let's cover the passives to paint a clearer picture of the vampires identity.

Savage Feeding Passive: After feeding, target is stunned and off balance for 2/4 seconds.
-This ability promotes feeding. It is, however, almost entirely useless. Take note that any target you feed on will not allow you to use Drain Essence on them. At base, your Drain ability stuns the target for 3 seconds AND heals you for 150% of it's damage while this just lowers your feeding level and angers the target. The only usefulness for this ability is in feeding [and I'm going to cover why feeding is almost entirely pointless].

Supernatural Recovery Passive: Increases Magicka and Stamina Recovery by 5%/10%.
-Great for mana or stamina users all around. Definitely recommended.

Blood Ritual Passive: Every 7 days, turn a player into a vampire at the altar.
-Make more vampires.

Undeath Passive: Increase damage mitigation when player is below 30%/50% health. Mitigation scales up to 50% dependent on players health.
-Absolutely mandatory. Until you get this passive. Fire will be your utter doom. With this perk you can even tank most PvE content and your healer will love healing you. This along with Mist Form is a very strong combination.

Unnatural Resistance Passive: Health regen is increased for vampirism stages 2-4.
-Very meh ability. Is significant if you always sit in stage 4 all the time.

Dark Stalker Passive: Move faster while sneaking and enter stealth more quickly at night.
-Nice ability. I wish there were more night vs day things for Vampirism than just a perk at night.


LAST BUT NOT LEAST
The reason why feeding is nearly pointless.
Stage 1 Vampirism: Take 40% more fire damage.
Stage 2 Vampirism: Take 40% more fire damage. HP Regen 25% Slower. Vampire Abilities cost 20% Less.
Stage 3 Vampirism: Take 40% more fire damage. HP Regen 50% Slower. Vampire Abilities cost 40% Less.
Stage 4 Vampirism: Take 40% more fire damage. HP Regen 75% Slower. Vampire Abilities cost 60% Less.

So let's get this straight. The hungrier you become the slower your passive hp regen is [still faster than skyrim by a significant amount] the cheaper your vampire skills are to cast. The trade off is by being hungrier, you can cast your INCREDIBLY STRONG vampire skills at less than half their cost for the trade off of slower HP regen. [Drain Essence totally negates this malus by itself]. Not only that, but this slower regen is only really noticeable outside of combat as all regeneration in combat is very low and with Drain Essence you usually finish a fight at full HP.

So the question, why feed ever? You should only feed if you remove your vampiric skils from the skill bar. That's it but then why be a vampire to begin with?

Feeding in this game is an incredibly poorly made mechanic and I'll cover the several things that if changed can make it an interactive and fun mechanic.

At present the problems are
Feeding vs Drain: Drain is always better, Feeding makes vampiric skills more expensive and only gives the benefit of reducing the HP Regen malus.
Drain Itself: competes with the Savage Feeding perk by already having it's own built in stun AND simultaneously gimping itself by being unusable on enemies with stun immunities.
The Hungrier your are the Stronger you are: This should not be a thing. This should not be a thing. This should not be a thing.


THE SOLUTIONS
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SKILL CHANGES
[Drain Essence]
-No Longer Stuns. Will interrupt enemy casting if they are not immune but always castable.
-While channeling, damage taken from the target being drained is reduced by X% [Unsure of balanced value].

Each stage lasts 1 Hour before promoting to the next stage.

VAMPIRE SKILL LINE LEVELING
No longer uses player EXP to level vampire skill line. Instead, Vampire skill line is leveled by feeding.

As for Hunger Stage 1 [FED]
-Gains night vision [at night and in dark places] - Toggleable.
-HP Regeneration is reduced by 25% in Sunlight.
-Vampiric abilities cost 60% less to cast.
-Take 20% More Fire Damage, Doubled in Sunlight.

As for Hunger Stage 2/3/4
-HP Regeneration is reduced by 50%/75%/90% in Sunlight.
-Take 25%/30%/35% More Fire Damage, Doubled in Sunlight.
-Vampiric abilities cost 40%/20%/0% Less to cast..
-Stage 2 gain short range detect life, Stage 3 and 4 increase range. Toggleable.
-Out of Sunlight, Vampirism skills are 10/20/30% more effective.


NOTES. The above suggestions traded power from Drain Essence [a vampire's bread and butter] for consistency and some lore friendly passives. Further tuning of the absolute strengths is definitely a welcome scenario and the numbers are only close approximations of theory before balance.

TL;DR
Please read it.