First I've heard of this, but then again we don't run Vista here.
Personally for something like this if your duplex and speed settings aren't showing a issue on the Riverbeds I would suggest just calling the support number and doing some captures with there tech guys to see if they can figure out what be causing a issue.
I recently have an issue where Vista PCs are not able to join Domain Controller using Steelhead. But when we took out Riverbed and tested without it, the Vista clients connect in a click to the Domain Controller. Can someone tell what could be the reason behind it? Rest all services and optimization is happening and all fine.