Mike Kolitz (Virtualization Team) has instructions on installing Vista into a VPC. To get VPC Additions working you need to use the Virtual Server 2005 R2 Additions (well, those are the only pubicly available ones that work). You can get the Virtual Server 2005 R2 additions from the trial edition if you don't have access any other way.

But what if you don't want to install Virtual Server just to get the additions ISO? If you run the following command, you can unpack the Virtual Server setup.exe file:

setup /c /t c:\temp

If you then run the following command, you can unpack the MSI file. And you then you have access to the VS2005 R2 additions ISO, which you can install into your VPC (just use the CD -> Capture ISO Image... option)

msiexec /a "c:\temp\Virtual Server 2005 Install.msi" TARGETDIR=C:\temp\extract /qn