Seems that in 3.60.04 (I know that this was working fine in 3.3) you can not extend VRayLight in a MAXScript plugin.
The following is all it takes:
plugin light LightTest name:"LightTest" classID:#(0x6237638f, 0x37546773) extends:VrayLight ( )
LightTest()
Trying to render then crashes with:
UNHANDLED EXCEPTION: Compiling geometry
Last marker is at ./src/vrenderlight.cpp, line 680: Preparing lights (LightTest001)