Ah well, I actually made the Vista partition active - some sort of warning came up about having to have operating systems on partitions marked as active, but I thought this would be OK. For all I know, that didn't do the boot manager any good either.

The XP partition was listed as active but the Vista one wasn't. I didn't attempt to run format, not when I thought everything was OK because I had no intention of formatting things. I tried format a lot later when I was attempting to resize the partitions.