I just swapped the transmission in my VT to one out of a VY (8HFD to 3HFD). Would I be able to get a memcal that will stop my car from holding gears for too long?

My advice would be to swap the all of the wiring including the force motors and other electromechanical items from your orignal gearbox to your new one. Otherwise we can't supply a tune to suit sadly