EasyC works just fine in Parallels. I have no experience with VMWare, but I would expect it would work just as well. As Kingof1337 said, Windows in Boot Camp does run the best, but at the expense of having to reboot. If you are alright with this, Boot Camp is probably your best choice, but I wanted to stay in OS X while coding. (Just so you know, I didn't use EasyC except as a WPILib reference, though I did open EasyC to see what it was.)
I would not recommend using Wine or Crossover Mac for robotics work. I've run the Microchip compiler in OS X using Wine, but anything graphical is likely to fail badly. Also, I would expect something crucial, such as the USB connection, to turn out to be extremely problematic under Crossover. (In case you do try Crossover,
http://www.codeweavers.com/support/w.../SerialOverUSB might help with a USB connection, but is more than a bit complicated.)
Lastly, there is a caveat with Parallels, although it is hopefully outdated and might not even apply for the Vex connection in the first place (I only know FRC). Back when I used Parallels during the build season, the USB support wasn't very good and attempting to use my Keyspan USB to serial adapter using Windows drivers didn't work well at all. Using the OS X serial driver,
SerialClient, and the "serial port" setting in Parallels did work, but was annoyingly complicated. However, things should "just work" with the latest Parallels. (hopefully)
And as Kingof1337 said, you need an Intel based Mac to do any of this. If you have a PPC, you've unfortunately got to get a newer one.