When controlling it manually keep in mind that the torque converter should not be locked up when the engine is under heavy load.
Say you are in third and climbing a hill it needs to come out of lockup, it will load the engine too much and make it over heat. That's what the vacuum switch does, as the engine load increases vacuum will go down shutting the lock up off. If you control it manually you will have to turn it off as you climb hills and the like.
If something goes wrong and the lock up is on all the time putting the car in gear at idle will kill the engine rendering it undriveable.