The sync signals from most VCRs are not good enough for a ReplayTV. The symptom is frequent blue screens with a "no video signal" message while recording from a VCR to a ReplayTV. This can be overcome by using a time base corrector (TBC), which produces a standard sync signal for the video coming from the VCR. Do a search here or with Google to find a time base corrector.
Aside from that, yes, manual recordings for each tape you want to record is about the only way to do it.
Aside from that, yes, manual recordings for each tape you want to record is about the only way to do it.