If the dryer is original, you definitely need a new one. The old desiccant is not compatible with 134a. The new models work for either R12 or 134a.

If you replace the dryer, you should definitely pull a deep vacuum before recharging. Remove the old dryer, and try to extract as much oil as possible so you can measure it. Put in the new dryer the larger quantity of what you got out, and what the FSM says for the dryer replacement, then quickly attach the dryer and start the vacuum pump.

You do not need to vacuum the system for a top-off of refrigerant, but you should definitely vacuum it any time the system is open to the atmosphere, no matter how short of a time it is exposed. If you're not flushing, try to keep the time that the system is open to an absolute minimum. The oil absorbs water and turns acidic.