Stu is right....
As other experienced owners tell you, 2400 to 2600 rpm for sustrained cruising. That's in your engine manual, too. We have owned a Universal M25XP for almost 20 years, and nowadays have a bit over 2000 hrs on it.
As to vibration, ALL diesels have some rpm ranges where they put more -- or less -- vibration into a boat. Often this is only a very limited range of a couple hundred rpm.
There is a lot of rotational mass, connected up and suspended in several ways, between the tranmission output and the the water being moved by your prop. And, going forward, between the block and the hull.
Starting with prop being checked for true-ness of each blade to its stated pitch, and balance, look to the shaft to be true, running through a good cutlass bearing. Center that shaft in the alley. Finally allign the coupling with the transmission.
On the other end of the situation, replace the mounts at (round numbers...) 1500 hours and 15 or 20 years. Both time and useage will degrade that "rubber" compound that is engineered to dampen vibration and support XX number of pounds.
Sounds more complicated than it is.... Just some preventative maintenance that goes with an inboard engine.
(Oh yeah... and don't go envying the guys with sail drives. You don't even want to know the yard time that goes into the scheduled replacement of the rubber membrane/boot that seals out the water.
"No free lunch" as the old saying goes.