>If the ISR vectors are for user programs, how does the OS handle interrupts that it needs to catch from slave 8052s?

You got me there :)
No, not really.
It doesn't. Except maybe of keyboard interrupts. With shared memory communication everything could be done asynchronously though. As much as possible in user's hands, there might be some OS routine calls by default on the ISR vectors (in low memory just LJMP for each), but generally these should be non-critical and overridable by user. That is, the OS should be able to survive without interrupts at all, but may make use of some (all?) optionally. I mean, there is vblank signal, there is RTC but if you don't need RTC but two timers, you override them with your own routines.