Right now any Windows 7 install you have will be based off the beta install and while I've heard and experienced nothing but good things about it, the beta does eventually expire. Once it does there is not a guaranteed way to migrate to the production version so it would likely mean a reinstall.
That said, XP will run fine in a VM particularly for something like Wavelab although you might see some issues if using something like a USB attached audio workstation or capture device. I don't have a lot of experience working with unusual peripherals through VM ware; it might be no problem at all. For simple work with .wavs .flacs and the like it will be fine. The other nice thing about VMWare is when/if you have to reinstall your base OS, migrating your VMs will just be a simple file copy then restart once you have VM ware reinstalled on the new host OS. One of the key benefits in a VM environment is the abstraction of the hardware. You could even move your VMs from Windows to Linux hosts or to other computers if needed.