If the necessary source files are limited to a small number of project trees, you could add a get step for each of those.
It's surprising that getting all the source code takes so long. Are you connecting to the database over a WAN or dial-up? If not, you may want to see if you can speed up your gets (archiving old history and ensuring you are not using the Contents file comparison method, for starters, or consider moving the database to a faster server or network pipe or migrating to a faster source control system).
There's nothing built into VBP to get just the files referenced in a workspace. Instead of the SourceSafe step, you may be able to create a Run Program step that invokes %DEVENV_NET% and invokes a command which executes a macro to open the solution, get latest, and exit the IDE (we don't use the VSS IDE integration so can't offer much help on this). Some links that may help:
http://msdn.microsoft.com/library/de...neSwitches.asp
http://msdn.microsoft.com/library/de...iveactions.asp
http://groups.google.com/groups?hl=e...3DN%26tab%3Dwg