A good place to start would be to look at tutorials for building 'combat robots'. There are some smaller weight classes that might be around the size of a robotic vacuum, however you could still look at the process for larger (100lb+) robots since it is generally the same, except with everything scaled down.
A simple base is just a wooden board, with 2 drive gearmotors for differential steering, along with 2 castors (or simply carpet sliders, those things used to slide furniture) on the other two sides to provide stability.