1996 Mercedes Benz E320 Vacuum versus transmission

Help! My MB 1996 E320 is acting funny. When traveling at Interstate speeds the RPMs are sometimes at 4000+ when they normally hover around 3000, and altered performance is noticable. You can certainly hear the engine working harder than normal. I emphasize sometimes. I can go a week or more without an incident. It is episodic. I took the car to a highly regarded neighborhood AAMCO (owner been in business for decades and reputedly very knowledgable), and he told me transmission was OK, but car was experiencing vacuum issues (whatever those are), and recommended I take to the dealer where I normally have the car serviced. Dealer then told me I need a new transmission. Who should I believe? The AAMCO would rebuild or replace the tranny more cheaply, but it would be odd to return to request a new transmission after they already told me it was working fine. If vacuum repair/replacement is fairly inexpensive should I do that first and see what happens? If so, do I return the car to my normal dealer/technician and insist on that repair first even though they have told me the transmission is kaput? I'm at a loss as to next steps. Many thanks for your kind assistance.