To save the pain from others of repeating my mistakes, I’ll write this short howto. In the end you should have a working system with Ubuntu, Vista and Dell MediaDirect.

1. Boot using Mediadirect CD, choose the second option and allocate how much you want for Vista (I gave the beast 50GB)
2. Install Vista
3. Boot into Vista and insert the Mediadirect CD again. Finish MD installation
4. Insert your Linux CD, boot
5. Install your flavor of Linux (I used Ubuntu Studio) by creating new partitions in the empty second partition.

After this I had a working setup with a working Mediadirect button (I could press the home-button when the computer was off and boot straight into MD)