It's pretty simple. We used this and it took about an hour on top to adjust our build system to label the packages properly:
http://www.add-in-express.com/vdproj-wix-converter/index.php
http://www.add-in-express.com/vdproj-wix-converter/index.php
There is no straightforward migration path from vdproj to WiX, either. It's a case of re-writing all of your deployment code (which has already been tested/tweaked/hacked on for years) for no upfront benefit.