Your answer is "psycho".
Now go.
Psycho eh? Go on. Tell me more of this "psycho." What does it do and more importantly, where can I find it?
Son, let me tell you, psycho is a rare and precious commodity. Nobody knows where it comes from, but by chance, anybody who sells drugs on the corner is bound to have some. It makes you fast, but it also makes you stupid. And all those arbitrary measurements of damage you take every time you get hit? Well, psycho makes it even more arbitrary. Save your game before you take it, cause you might get junkie disease, which is bad.
Nevertheless, one dose of the stuff will make you just about impervious to bullets of the 10mm variety, you'll take 0, 1 or 2 points of damage each hit, and it'll allow you to take more of any other kind of damage, but you won't be invincible. For example, in the Den there's a bar next to the Slaver's Guild. Punch the barman in the face until he is dead. On his body, you will find a syringe full of precious, precious psycho. Then walk into the Guild, take the psycho, and become their daddy. Take them to school in a van full of pain. Be forewarned that unless you hook up whoever it is you're with, they'll be vulnerable to all the bullets bouncing off of you. Repeat where and when necessary. It don't pack quite as much power later on in the game, but once you get your power armor it becomes obsolete anyhow.