If you earn about 120.00 per hour take your car to mechanics. To pay 60-80 per hour out thats what you have to earn. If not like most of us learn to get involved. The more you do the easier it gets as well. Self confidence grows right along with your efforts. It is one thing you really cannot purchase in this world. We all had to start somewhere.
You have a tremendous advantage with the existance of this excellent site. In my time auto repair was almost a self learnt thing totally. A mity vac is a very small price to pay in the overall cost of what you might be facing. It even has residual value as you can later sell it on ebay later if you want.
You can also prevail and find out if the mechanic knew what he was talking about. Opinions are just opinions in general without verification. He may have been right but why roll the dice? I almost bet he did not check the vacuum out before giving his opinion.
Roll the dice too many times in life is like scratching in pool games. You just plain loose. Your mechanic may still land up with the job as doing internal things to an automatic transmission is for the really experienced few. By testing the external systems yourself at least you will be sure it really needs it. It's all in the archives. Or ask questions as you get started. Many on site members have a lot if not vast experience in this area. I also believe you can do it or would not suggest it. The clues were in your post. I also know you were already thinking along this path. So I thought to boost the ideal up a notch or two for you and verbalise the rational for it might help you as well. Or at least try to explain somewhat how I view these things.