Hi. In addition to owning a Vista machine I also have a Macbook. I noticed that the games from this site offer older games for XP/Vista (hence why I was so eager to buy Fallout and Beneath a Steel Sky) but is there anyway to get them to work on a Mac?

Sure: VMWare Fusion, Parallels, or CrossOver Games might work. Look into Darwine as well if you want a free solution. Those are all different (but related) solutions and if you're not familiar with them, it'd be easier to just google each one to get an idea of what they do.
I just picked up the Fallout series and I'm running Fallout (first one) just fine in VMWare Fusion 2.
Worst case screnario, XP or Vista via Bootcamp should work just fine.

At one point Fallout 1 and 2 were ported over to Mac OSX. I have these versions, and they still run well. I would love for GOG to provide the ports as it would great to be able to pick up a game for the PC but also have it available for the Mac.
You should be able to run Beneath a Steel Sky on a Mac by using ScummVM. I haven't tried this with the GOG version but you should be able to point ScummVM at the data files and have it run. The only problem you may encounter is running the GOG installer, which is a windows executable. As kingincrimson suggests, Wine or Crossover may help with that. If your fairly Unix savvy you want to look in MacPorts over Darwine however, as Darwine is pretty out of date.
Hope this helps.