Can I use Windows 7 boot DVD or Windows XP CD to restore
bootmgr under Vista? I don't have any DVD for Vista and it cannot be started. Or is there other way to fix it? Also would it matter if it's Win 7 x64 repairing MBR on VISTA x86?
You should be able to use the Windows 7 DVD okay. Windows 7 and Vista both use the same bootloader so there should be no problem.
Using the Windows XP CD would install and older bootloader which will not be able to boot Windows Vista, so that won't work.