1. Settings->Configure Mail and Click on Composer
2. Click on "Use External Editor" and specify the editor as "xterm -e vim -f %f"
3. Click Apply.
Open the composer and type in mail id and subject and click on the body. When you start typing a new editor window opens up which is vim. Type in your message and type ":wq". Thats it.
For people intending to send patches, use :vsplit