Look at the specifications of the Fairphone. It features an E-Compass and thus should have a magnetometer.
However the E-compass has been reported to be not fully reliable:
I do not own a Vaavud Sleipnir and it seems that it is not yet available to buy, so very probably nobody will have any experiences yet.
Maybe the manufacturer can give you a right of return, in case it does not work with the FP?