If you turn the active bit off for Vista before you install W7, it will put its bootloader with W8 (and not auto dual boot it with Vista. You can then boot W8 by default and add a Vista entry. The BCD in Vista will remain separate and unchanged.
But as CG says, it's not important. Vista/7/8 all use the same MBR/bootmgr/BCD. It's just the release number gets higher. Older can have problems booting newer because of digital sigs, but you should have no problem with the newer version remaining behind.
You never need to rewrite the MBR unless a different bootmanager (XP, Linux, 9x) has altered it.