You can install a boot manager like EasyBCD.
Download EasyBCD 1.7.1 - NeoSmart Technologies

Or you could change the boot order in the BIOS (setup).
Most newer systems also have a key, like ESC or F8 etc, that can be hit during the POST to choose a boot device.

Or you could put Vista in an external enclosure, set the first boot device to external, then simply unplug or turn off the external when you want to boot to the internal.