Well,....I spent 8 years as a transmission man (until I wised up and got out from underneath other peoples vehicles)
The problem with diagnosing a vibration without having the car there in person is that there are SO many things it could be, and everybody's description of the vibration is going to be different.
How many miles on the car?
Is the car an automatic, or standard?
If it is automatic, is it shifting too early or late? (it can "chatter" if it shifts to early, sort of feels like when you let out the clutch on a standard to early)
Are the motor mounts okay? (if the mounts are bad "particularly on a front drive" the torque of the shifts will cause the motor/tranny to rock back and forth)
Have you recently driven anywhere where your car may have contracted a case of gremlins?
These are just a few of the possibilities
Its really hard to say without driving and examining the car, take it to a reputable wrench, and hopefully he can help you.