If you don't have access to an instrumental, you can do it with a mix of EQ, sometimes noise reduction, channel converting, and playing with the sampling rate. Sometimes when the vocals are predominantly in the center of the mix, you can remove a majority of the other sounds without affecting the vocals with a channel converter. I've never gotten that to work perfectly though and, more often than not, find it doesn't help much. For isolating hip-hop vocals, the best results I've gotten are by dropping the sampling rate to 22.5khz, doing a heavy bass roll off, and boosting the mids around the vocal range. There will still be some chatter from the original track that may be noticable. But, for the most part, it works pretty well if you have no other means to grab the vocals.