With VemsTune previous to 2010-08-19 I had trouble downloading SD card logs, or even getting the overview of the logs on the card. Try lowering the communication speed for the download (Prefs -> ECU comms) if you still have trouble with the download halting mid-way.
I have a race coming up in 2 weeks where I will be working with 3 of my cars with SD Logging. I haven't had the chance to try the lower comm speed, so i will try that and the 8-19 VemsTune and report the results. I'm on a Windows XP laptop and a regular Serial port.
I had a car on the dyno and the SD card workes flawlessly at 57,600 baud rate. I'm happy.
The VemsTune 8-11 screen refresh rate is having problems though, the "Main-Text" screen was the only
screen that was even remotely useable when reading the lambda values during acceleration.