I've read that there is a bug in dawnguard, where if I learn the Drain vitality shout before the marked death there will be a glitch.

Was it fixed in recents patches ?

Should I avoid completing this part of dawnguard before I get all of the three ?

Where to get them lol!

PC or console? if PC you can fix with command line, if on console from what I read you probably better to disable dawnguard, learn all 3 words before installing dawnguard. or if you don't intend to learn that word you can just ignore it

Just get all 3 words for Marked for Death. Look it up on the wiki or the UESP.

Autumnwatch Tower
Forsaken Cave
Falkreath Dark Brotherhood Sanctuary

I'm on PC. Should I really restart to my save before I go there ? cause I just finished that dungeon... but I a save right before.

annoying bugs.

I'm also reading that having dawnguard installed in the first place can mess the whole thing up.

After 35 hours invested into it I do not want to restart again, not for now at least.