I know in game that it is possible to not feed at all even if your a vampire, of course you will be stuck on stage-4.

However in lore, is feeding for vampires something that is necesarriy for survival? In other words, if they don't drink blood they die for good?

Or is it more of a mental thing? Like an alcoholic's urge to drink, or a junkie for drugs. If that was the case, then maybe feeding does make them stronger, and they have the urge to do it, but they don't need it to simply exist (in other words if you lock up a vampire for a certain amount of time, it won't die because of lack of blood, could become weak though, maybe instead of dying it would go crazy).

If vampires do need to drink blood in order to live, then why wasn't that implanted in skyrim (so if you don't feed at all for a certain amounts of days, your character will die)?

in morrowind I cant remember because its been so long since ive played it extensively gameplay wise. I rp my vampire so I avoid sunlight and generally feed regularly

Have you played Oblivion's "Origin of the Grey Prince" quest? The answer is: no. At least, not for a good 30 or so years. However, he did go insane from his ravenous thirst...

Lore wise they go insane from a lack of blood but they can live without it. (see the Grey Prince's father in Oblivion and random vampires found in caves that yell that they NEEEEED your blood)

So technically they do need blood to live if you consider living being in control of all your faculties and having the ability to think.

If those last two things don't count towards living no they don't need blood to live.

I think some need to feed while others do not.

For the same reason eating to survive as a living being hasn't been implemented.

They don't need blood to survive, because the Skyrim strain of Vampirism is different from the other strains and yadda yadda yadda. Loop holes, plot holes, worm holes, etc.