The symptoms show up as an error message dialog with code 2869:
This is apparently some sort of permissions issue. To solve it, the MSI needs to be run as an administrator. The easiest way to do this is to create a bat file to run the msi manually. It would be something like this:
msiexec.exe /i "c:\SoapBoxServer2007\files\SoapBox.Server.Enterprise.x220.127.116.11.69.msi"
Then you right click on the bat file and choose "Run As Administrator". Presto, a working installation in Vista.