It's really bad man, take it from me!
Ideally you should just get powder, but since I doubt you're going to drop your crack and go blow some more money which you might not even have, then I would recommend using the citric acid packets that they often have at needle exchanges. beyond that, I've found that Emergen-C works pretty good (a lot better than vinegar), though this shit still does a number on your veins. You really want to be careful, make sure that you stir the crack around a lot and that it's dissolved. I once came close to death because I wasn't adding enough citric acid to my shots, and in return not getting the bellringer I was so invested in. I had kept adding more crack to the same spoon, and keeping the residue each time, which seemed to be a lot more than usual left over. I stupidly thought to add a shit load of Citric Acid to my last shot, which effectively did dissolve the residue. I then injected it, and to my surprise, I became deafened by the ringing of bells. However, instead of being dropped back down after two minutes like normal, I kept getting higher and higher until I could barely see or move. I was stuck on my floor for nearly an hour, wanting to prep the two bags of heroin I had bought for the crash, but unable to due to the fact that I feared my heart would explode if I moved a finger.. Very fucking scary.