Vagrant doesn't make it easy to manage multiple versions of it. I was lucky enough to ended up working on a project whose vagrant up only works with the legacy Vagrant 1.4 and another one that only works with newer versions. I'll explain here how I managed to keep both. In this case I'll install Vagrant 1.4.3 and 1.7.4 (the newest as of now), but in principle it should work with other versions as well.
1. Install VirtualBox 4.3 and Vagrant 1.4.3
Vagrant 1.4.3 doesn't work with newer versions of VirtualBox (>4.3). During this experiment I messed up all my installations several times, so I'm installing everything from scratch (just go to the download page, download the dmg and run the pkg installer normally).