So this appears to be my issue, it looks like I've had this setup wrong all along. (I've only used is as a reference before the last few days) The arguments should be:
vsmerge [options] %base %mine %theirs %merged
Sorry for my confusion.
The /h command line argument for vsmerge in v18 shows help while it doesn't in v19. I couldn't find any argument information in the help files.
~Tom