You can do it only for certain stereo tracks. If the vocals are exactly the same on both stereo channels, you can remove them by “subtracting” one channel from the other. You can do it with many studio recordings, where the vocal track is mixed exactly in the center. In order to do it in Audacity:
Import your stereo file into Audacity.
Open the track menu (click the arrow next to the track title), and choose “Split Stereo Track.”
Select the lower track (the right channel) by clicking it in the area around the mute/solo buttons.
Choose “Invert” from the Effects menu.
Using the track menus, change each track to “Mono.”
If you are not satisfied with the result - the vocals are not exactly the same on both stereo channels, you can use other techniques.