I've been using the Fantom under 64-bit SONAR since X1. In fact, I didn't even realize that it wasn't officially supported until I was recently downloading updates while getting it working in X2 (x64) on Windows 8.
When using it in 64-bit mode the VS editor will load using SONAR's built-in Bitbridge. The only issue that I have (and I'm not even sure that it's x64-specific) is that you have to start playback a beat or two early or otherwise the first MIDI notes will not playback. Otherwise, it works just as I expect it to.
BTW, I also have an ARX-01 drum module installed in the VS700R I/O module and that works too....usually.
-e.B