Thanks for the responses. I worked on it for a while this morning.
It must be something in the way Boston recorded their music, because I can eliminate the vocals from other songs, but not any by Boston
Instead I think I'm just going to loop some of the guitar parts that already don't have vocals. I don't need it to be terribly long, I just want to use it as theme music to something I'm going to do for OSU's student television channel.