I have figured out how to get things workable on my set up. When editing the path map, I selected "Path in File System" rather than "Path in Workspace".
This was successful in giving me the following option when the debugger opened: /RemoteSystemsTempFiles/LOCALHOST/Users/xxx/Dropbox/joomla-vagrant/www/test.php
-- So, it must have been something with the way my workspace is configured.
--- EDIT ---
After using this setup for a couple of hours, it too broke. What I ultimately did is just start a new workspace from scratch and so far everything is working correctly