Some progress. Took another stab at using Repair with just the one disk. I believe that system is now booting reasonably. A bunch of paths are wrong for that system (e.g., the Desktop is still from C:\users, but key settings like WINDIR are now showing as L:\windows when booted back in the multidisk environment). This surprised me, I thought the system would switch C: and L:, but they are still definitely lettered the same (verified via label files I created). I hope this doesn't create confusion when I try to do the Windows 7 update test on the L: system -- I don't want to blow away the MBR or anything else associated with the C: Vista system.