Although 1 GB is sufficient enough for what you want to do, I would HIGHLY recommend 2 GB for any kind of multiple OS setup with specifically Vista.
Mac OS X runs beautifully with only 1 GB of RAM. XP runs fine with just 1 GB of RAM. Vista... well doesn't. To me, 1 GB for Vista is a minimum, not a recommendation.
With that said, in the case of a multiple operating system setup, Boot Camp allows dedicated hardware when running Vista. Even then you will have only 1 GB of RAM to run Vista. Even on Windows-based hardware, 1 GB is pushing it. I would recommend 2 GB.
If you are running any kind of "OS within OS" software (ie. Parallels or VMware Fusion) just remember that you will be running TWO operating systems at the same time. Vista Alone, at idle, uses about 60% of your available RAM if you only have 1 GB. That number drops to 30% if you have 2 GB. Imagine Vista taking up 60% of your RAM, and only having 30% to run everything else, including Mac OS X.
So... To answer your question, yes, 1 GB is sufficient to run Mac OS X and Vista on the same machine, no matter how you decide to run Vista. However, you may find yourself running into RAM shortages, especially if you are running Vista within Mac.
I recommend you opt for the 2 GB.