It's just a forward/backward in/compatibility problem. Each newer version of bootmgr will boot an earlier Longhorn (W8 will boot Vista/7), but the older versions don't recognize the sig on the new loader as "genuine Windows".
If you install W8 with Vista/7 visible, it will take over the existing boot files and replace them, making the multi-boot automatic and transparent.
If you contrive a multi-boot where it didn't get the opportunity to do that switch for you, you must either boot from W8 and add the older systems to its BCD, or manually upgrade the bootloader on Vista/7 so that it doesn't think W8 is bogus.
If you get tired of W8 and remove it, you can revert to the Vista/7 version, but you don't need to.