Now thats a strange question. Maybe you can, maybe you cant. I havent tried and i wouldnt for that matter. Why would you not just use the win7 install disc ?
I don't know, xp can't do anything to vista/7 bootloader. In MS config however you can change the settings showed there (like the time to chose between dualboot os etc) from within vista and windows 7. I don't say all, but most of the basic settings can be set also from vista.
Of courae they both would use boot manager. That is not to say that the Vista loader is the same as the Win7 loader. I would just use the Win7 disc to repair Mbr. It makes sense to do this.
Of course they are diferent, "vista" - "win 7", but MBR only point to a file, in both cases bootmgr, after the boot kick the file bootmgr load the actual SO.