The 79 Diesel does not have a vacuum modulator. It has a fixed modulator. I have never had one that adjusting made any difference.
To get shift strength on that trans you will need to make it shift later with the mechanical linkage. (shorten)
About the only thing that might help the slow engagement would be if the filter is restricted. A new one might help. Probably nothing else simple. You might try an additive designed to soften seals when you change the filter. We use a product by "Lubeguard".
Bosch Master, ASE Master, L1
33 years MB technician