Trying to start my mission for the Brotherhood but I can't find any Rad-X anywhere, as the glow seems to irradiate me faster than I can chug radaways.
Searching online reveals it to be sold by vince, who can only be talked to after an 'introduction' I rebuffed the bum earlier on, in the game and now he refuses to talk to me :(
Apparently jake the arms dealer is willing to introduce you if ask about radiation, however that option isn't available. Solutions?

Murder is always an option.

Unfortunately murdered drug dealer loses all his supplies on death :( It's also seemingly impossible to steal from him as dialogue is initiated on entry to his shack.
However I've just discovered that you can barter from inside the dialogue window, by pressing the big red button, instead of having to initiate it in conversation.
It feels a bit weird being told to leave immediately and then spending 5 minutes trading with the guy, but it works!.

Oh hahaha, I didn't know you had a dialogue window, yeah, If I knew that I could have told you right away.

go south to the bottom of the map and find the cathedral.
theres a monk guy thing who sells stuff. In any case, go in,
kill everyone and everything (dw you might as well, because you have to kill them later when you begin the attack on the master who lives under the cathedral)
after you killed the noobs on the ground level, go to the table the monk was next to.
use skill "3" steal - and you should have access to everything you need, radx, rope, geiger counter, rad away, mentants, buff out etc. then go go glow, stop about 3 squares away from glow. take rad X, go into glow, use rope, get disk + key card, get back out. go see brotherhood guard, and you're set :D

There a couple guys in The Hub who sell Rad-X. The general merchant as well as a guy in a leather jacket in old-town both sell it. Best of luck