Hmm, in that case, maybe it would make sense to have model direction between (-\pi/2, \pi/2), Something like:
angle = pm.VonMises(...)
direction = tt.stack(tt.cos(angle/2), tt.sin(angle/2))
coeffs = pm.Normal(..., shape=nvec)
vectors = direction * coeffs
So that angle is still continous between -90º and 90º