If what you are saying is,, the vibration doesn't go away with speed changes then it might be the road you were on. Just let it go until it happens again then analyse the problem.
As for the gauge mine has always run on the low side. Thru many a thermostate and other cooling system change, that's just how it reads.
One easy check for you is to put a thermometer in the radiator when the engine is warmed up. (I'm sure you know the hot engine/radiator cap removal thing.) Then relate the temperature to your gauge on the dash. I think most older cars run best at around 180 degrees?? Maybe someone will chime in with their opinion.