Actually, something like the flow plane (the plane in which all particles move) only exists in simple shear, and the foliation planes in the rocks you study will only be parallel to the flow plane if strain is infinitely large. The vorticity vector is related to the spin tensor which is used to describe the average angular velocity of material lines passing through a point. Adding spin tensor(s) and stretch-rate tensor will result in the velocity gradient tensor which can be used to describe any general flow. From all this it follows that  the vorticity vector can be oriented oblique to the foliation, which images the XY-plane of finite strain. I am not sure that my answer points you in the right direction, as I don´t know particulars of your analysis.  

I understand "the foliation plane" in your question as the shear zone boundary like the C-foliation. If you are considering steady-state flow fields in tabular zones, the principal strain rate axes are fixed relative to the zone boundaries and the vorticity vector must lie in the shear zone boundary. If non-steady flows are considered, then the internal vorticity vector may intersect the shear zone boundary. Now, it is generally understood or assumed that the C-foliation is a plane toward which material line and plane elements rotate. The C-plane itself has no rotation relative to the flow principal strain rate axes and therefore the vorticity vector lies in the plane. Of course such an assumption is subject to test.

see Talbot C.J., 2014b Empirical Paths of Poles to Planes (eppps) in shear zones. Journal of Structural Geology, 66, 309-333