it's cheating only because of the limit of 1 gyro. when the rules were written i'm sure noone thought of putting gyros on the head. so, 1 gyro limit was probably in the spirit of keeping one gyro on the tail.
maybe you're forgetting that the vbar is a virtual representation of the actual flybar. there is no competitive advantage to the vbar as far as stability goes. it's simply doing electronically what the flybar already does.
there is an advantage in rotational mass, limits of pitch, and control over phasing, interactions, etc. but, that plays no part in why it would be considered cheating. you'll have no advantage, for example, in flying a vertical hexagon with one system over another.
if it ain't broke, break it.