I originally had my HSync and VSync combined when connecting my ArcadeVGA to my WG25k7191. Originally, my PC/VGA Act Labs USB Light gun didn't work. I recently read a post from Andy Warne that said he did a lot of testing, and determined that the Light Gun won't work unless you separate the VSync and HSync.
It meant that I had to add a wire to a contact on my monitor that never had a wire before (for the VSync). Now the Act Labs gun works great:

That's one advantage of splitting them, anyway.