Turns out that this didn't solve the problem at all. In fact, the only-homepage modules are showing on a bunch of new pages now.
I don't understand what an itemid issue is - my categories have ids, but there are 8 level 1 categories and nothing above that because that's how my navigation works. In the Module Manager, mod_virtuemart is disabled altogether, but enabling it doesn't solve the problem, and creates problems with my layouts.
I'm so confused. Clearly not too smart, either.