I use two systems, one with Win7 64 and one with Vista 64. My Cubase project uses an instance of VEP locally and one on the remote system. It all works fine when composing and playing back even with 6150 and Cubase 5.5. If I export at real time it doesn't lose anything off of the front of the song. If I export faster than real time it loses the first 3-5 seconds of each track.
In my case I use MEAP to export one track at a time have this problem, so I know it's not too much data or anything. The export starts from the beginning of the midi track but you can see there is no meter movement in Cubase or the VE Pro mixer until it is part way into the export. At real time, the meters begin immediately and you don't lose anything.
I even tried just exporting one of my tracks manually and moving the midi data farther down but still starting the export area at the front of the timeline so as to give it a few seconds to start up -- but it still loses the first part of the music. I just assumed this was a Cuase 5.5 issue. But now beatpete makes me think it was introduced in the new VEP pro. For me it's just an inconvenience now.