We've finally released the Microsoft Virtual Machine Additions for Windows Vista Beta 2!
You can download the additions here: http://connect.microsoft.com
Log into the site, and sign up for the Virtual Server 2005 R2 SP1 Beta if you're not already in it. In the downloads section, you'll see the Additions for Beta 2.
To install them run the MSI installer from the download site (yeah... that should be pretty obvious, actually). The installer will ask you where you want to put the new Additions, but will default to %SystemDrive%\Program Files\Microsoft Virtual Machine Additions.
To install the Additions in your Vista VM, just mount the VMAdditionsforVistaB2.iso file in your Virtual Machine, and run the installer as normal.
Update - It's been confirmed that these additions do not work on the Beta 2 Refresh that was released recently. We're looking into this.