So set sort by Category then Vendor. Or sort by Vendor then Category.
Those are easy one step fixes, that give submenus.
How do you have sort set in Nuendo?
Vendor?
Vendor then Category?
Category then Vendor?
Maybe there’s an infinite vertical scroll in Nuendo, but there are no other workarounds in Wavelab 7, as far as I know. You could try a demo of Wavelab 9 to see if it’s any different for you.