Although you changed the MTU size when you saw the terminal-services stop working and the Internet connection start working, I don't think this was the direct cause for the change. It's more likely that one of two things happened.
First, you need to make sure that you have "split-tunnels" enabled on each SonicWall; split-tunnels is the capability that lets you simultaneously surf the web and use the VPN. Second, it sounds as though you may have inadvertently disabled the VPN when you made the change to the MTU size.
So, it probably makes sense to reset to the original MTU size and then check the split tunnels setting.
Hope this helps,
This was first published in September 2002