Hey Gijs,
Just a thought but as spotlights seem to be round and therefore do not care about any possible rotation in the plane perpendicular to their light vectors - can you not create two planes from normal using the ‘from’ and ‘to’ vectors and then do a transformation plane-to-plane?
https://developer.rhino3d.com/api/RhinoScriptSyntax/#collapse-PlaneFromNormal
https://developer.rhino3d.com/api/RhinoScriptSyntax/#collapse-XformRotation1