I'd get rid of all the little holes, and the bottom plate. Run a single vac line from the motor to the center of your grid board, and seal off the unused portion with a foam rubber gasket. A vacuum cleaner motor has low vacuum, but high volume, so you want a large tube to move as much air as possible. You might want your grid grooves to be aligned radially to the hole, for better air flow back to the tube.
3) You'll probably find that your motor is barely strong enough for holding your parts, let alone doing both workholding and dust collection.