I'm considering contracting the disease with my rogue (unsure if it's worth it), and I'm curious; when feeding on NPCs, does the act of feeding on them infect them as well?

Also, does feeding on them kill them?

Lastly, is becoming a Vampire without a mod at all worth it for a thief-ish character?

No, it doesnt affect them
No, it doesnt kill them

And well most of the Vampire buffs are related to being sneaky, though Ive heard from players it doesnt really help much

For it to be worth it, do you need to degrade (so the speak) to the 4th level of ugliness? Where people hunt you down? I looked over the abilities, and they're quite bad, until you get to the 4th stage. Is this correct?

It's really not worth it unless you are an illusionist who doesn't want to dual cast since it boosts the level of enemies you can affect by 25%. The fire resist weakness kinda svcks, unless you are a dunmer.

I became one for the sneak bonus, but after awhile it doesnt really make a difference. Also feeding becomes an anoyance very quickly.

You can always order your follower to sleep in a bed and then feed. There's plenty of beds in most caves or dungeons ^_^

After biting the majority of the Royalty including the ghost of the mad king it became boring.
The only real difference is that you are weak during the day and that in the last stage the game becomes unplayable if you stay there.
Also your vampiric abilites vary between the stages and so constantly get kicked out of the quickbar.
It could be nice as an Illusionist though.

blood should give you power the whole system seems backward to me and vamps should have a unique power\shoot like using blood to reanimate dead more blood means stronger corpse means higher stage vampirism

Def not worth it. I would wait for a mod to change the buffs and system to be fun and interesting.

You should be able to kill by feeding too much. You should also be able to create followers. Also get weaker the longer you DONT feed, they have the whole thing backwards again!!