I hate Macs but I hate seeing someone who was smart enough to get vista going on a mac with no issues having issues switching back and force so I'll help ya out.
First of all, there's a number of issues installing vista on a mac, luckly you didn't run into any of the install issues, but you are having the issue of switching back and forth and in order to fix this you will have to put in your Mac OS X Leopard DVD in your DVD drive, and install Boot Camp on your now Vista computer. Once you installed Boot Camp on your now Vista computer, you'll have a new icon in your system tray for your ugly Boot Camp from which you can set your startup OS prefence and what not and, oh look at that, you're now able to get online as well...
Doing that "should" also install the needed drivers for getting your wireless working as well but, as I said, "should".
Using Boot Camp to install Vista on your Mac: The Complete Walkthrough - Simple Help