I don't think that dihedral will have the desired effect here. In a normal plane, when it rolls a side slip velocity component is added to the free stream which changes the angle of attack of the wings. The upper wing's angle of attack decreases, and the lower wing increases, which changes their respective lift and forms a righting moment. While you will experience some side slip due to force balancing on you saucer, it will not have a stabilizing effect because your lift is not created by airflow over surfaces. Your fans remain a constant distance from the CG and a constant angle relative to the CG, so the moment they create about the CG can only be changed by varying their thrust. Putting in a dihedral angle to their thrust will have no effect, except for requiring more thrust for level hovering due to the thrust not being perpendicular to gravity. I drew up a free body diagram of your craft, and found a very interesting result. As the craft begins to tilt, the moments contributed by gravity out by the fans actually increase the tilting. The lower wing's moment is a function of cos(phi-theta) and the upper cos(phi+theta), where tan(phi) = distance cg is below fans/radius from center of craft to assumed point of gravity acting, and theta equals tilt angle of craft. As can be seen from the two cos expressions, when phi is greater than zero, i.e. the cg is below the fans, a slight tilt increases the downward moment on the lower wing and decreases it on the upper wing, which augments the problem. This seems counterintuitive to me, and it's effect is probably almost completely wiped out by the pendulum effect, especially since a very realistic assumption for your craft is to assume all the weight is clustered right around the CG. If anyone draws up a free body diagram and disagrees please let me know, as I have rechecked mine a few times because it is so counterintuitive, but can't find a problem with it. I agree that your stability issue is much more a controls issue, since changing the thrusts will have a huge effect on angular acceleration of the craft. Why did the Osprey have problems? vertical thrust applications are hard to control well because they don't have much natural stability.
Good luck with your project.

Jamie