Wait wait wait. It sounds to me like suezeekay already has all the equipment she needs, except for a simple RF splitter/combiner, assuming the modulator she’s using with the satellite box and the modulator built into the VCR can be set to operate on different channels (most let you choose between 3 and 4, and some let you choose almost any channel). All you have to do is set the modulators to different channels and then get an RF “splitter” (which is a combiner if you use it backwards) from radio shack. Plug the coax outputs of the two modulators into the “legs” (generally labeled “output”) of the splitter, and plug the other tap on the splitter into the TV. Voila, your satellite box is on one channel and your VCR is on another.
A note on splitters/combiners: there will be two identical-looking products at radio shack, but one will be, say, $4 and one will be $7. You want the $7 one. It’s not a scam – you can definitely see the difference in passed-through signal quality, and it really starts to matter when you’re combining lots of signal sources.
If neither modulator lets you pick a different channel, you can always just replace one of the modulators with one that does (you can even effectively replace the built-in modulator on the VCR by using the VCR’s composite video output and buying an external modulator).
suezeekay: if you do plan to go the switchbox route, double check the switchbox you buy to make sure that it can actually switch between composite (RCA) video and RF (75 ohm coax). Some (I’d venture to say most) boxes don’t do what you might think. They’ll have it so you can have RF output hooked up, and you can switch between the 3 RF inputs, or you can have composite output hooked up, and you can switch between the 3 composite inputs, but you can’t select an RF input if you’re using a composite output. (Geek talk: if you think about it, a switch box that allows converting from one to the other would require complex configuration: when converting RF to composite, it needs to know which channel to decode, and when going the other way, it needs to know what channel to modulate onto).