If you want the smallest file size, do a rough stereo mp3 mix with a guide vocal, and send that through regular email, along with the BPM of your project. You would want to receive the vocal back as a 24bit wav file/files. The person sending the file back to you just needs to make sure that they extend the blank beginning of the vocal clip/clips to the very first bar, so it will align with your project. it's just a matter of importing the vocal/vocals into your full project.