Your SDL should have a 722.321 transmission but I think you missed the point of the advice you received. There is no vacuum system in the transmission; rather, the engine vacuum system sends a signal to the transmission. More vacuum = soft shift, less vacuum = firm shift. For a consistent shift feel, you want more vacuum at low engine speeds and less vacuum at high engine speeds.

The 86-87 SDL has a more complicated transmission vacuum signal system than other MBs of the era. Instead of only a vacuum control valve attached to the IP, there's a vacuum amplifier that considers as well coolant temperature and intake manifold pressure. The vacuum amplifier is often called the blue fyling saucer (BFS) in the forum archives. The 87 300D and TD and maybe some of the 190Ds have the same system. Other MB Diesels are content with just the VCV.

The first link in this post has the vacuum diagram you need - http://www.peachparts.com/shopforum/723828-post1.html

Sixto
87 300D