You have two options:
1) do as you suggest, and switch boot order on power up, or
2) set up Vistas bootloader to give you the option of booting vista or Lin ux.
search on vista/Linux dual boot. You should turn up instructions on how to configure vista bootloader.
When you install Suse to the external drive, install grub to the root partition, and not to the MBR. "root partition" is the boot sector of the Linux partition, not the MBR of the external drive, unless the instructions for setting up vistas bootloader to boot Linux tell you otherwise.
My personal choice of bootloader is grub. I've used both grub and lilo. I find grub easier to work with.