Just throwing this out there, but you could always do what I did and purchase a combined GPS/Compass from a Naza "Lite" and mount it externally. It's 100% compatible and there's simply no interference this way. Just remove the internal GPS and compass and plug in the puck. The assistant may ask for an IMU calibration, and you obviously need to do a compass calibration, but after that it's all smooth sailing.
GPS locks are fast, tight and stable, and the compass works beautifully. As you can see, I have an aftermarket landing gear which I modified to fit a P2V+, but there's no need to worry about compass mounting on the leg because it's all part of the combined GPS unit.
I went the elaborate route by snipping off the stock molex on the puck's cable, shortened it, and ran it through a grommet in the rear of the top shell, and then soldered on a new molex. That's much more elaborate than necessary though, as one could simply make a small cutout in the top shell and use the entire stock, intact cable without modification.
On the vision (original) there's only the one GPS port. On the Vision+ there are separate compass and GPS ports, but the GPS port still accepts compass signals through the GPS port, so it works fine on either flavor and probably works fine on a non vision as well.
Naza GPS/Compass Puck $100:
http://www.banggood.com/DJI-NAZA-Lite-V ... 86239.html
Perfect GPS locks - priceless.