Tom, I've always understood that a UL fork is the same as any other "big twin" fork and yes, the front end parts are interchangeable between all "big twins". To my way of thinking (always slightly suspect!!) the VL isn't classified as a "big twin". The VL fork does differ in a few ways. As I recall, the spring rods and lower springs are longer on the VL and the front spring fork on the VL is a little shorter as well. However, the front spring fork, spring rods, and lower springs will interchange when used together as a unit. I say that because I'm pretty sure the fork end rocker stud diameter is the same (this is something (rocker stud diameter) easily changed unless your a purist), although I'd say swapping rockers has more to do with the type of axle, hub, and brake assembly your using than the type of fork.The top clamps do not interchange between VL and big twin (different size holes for top of leg tubes and stem nut assemblys).
Regards,
Geo.