Vibration at idle

check front end by starting car have someone rock steering back and forth look for any parts that is moving more than others. keep in mind high speed vibration can be tires, wheel bearing, trainning and mounts. have someone follow you and looking at tires for bearing if both drive tires are shaking look at training and steering. good luck

I believe this vehicle is equipped with a dual mass flywheel. It may be starting to seperate causing the vibration

check your tyres sounds like buldging in the rubber replace immediately

possible wheel bearing at fault. if it heard/felt more when turning or swerving and is a growling type noise, more than likley is. possible bad tire shouldnt be over looked. is abs light on? most often when wheel bearing is bad a speed sensor ( within wheel bearing ) will go open, setting abs light. vehicle needs to be ran on a hoist to try and duplicate noise/vibration.

check your spark plugs. it sounds like you arent getting that great of a spark

check engine and gear box mountings

Yes it can but the dealer would have found it on the test drive if they test drove the vehicle. The only other causes for the vibration I can think of should have been found on inspection as well. Check your bushings and mounts for the sway bar and your tie rod ends as well.

Front shocks are week

Vibrations while driving like you are describing can come from tires that have defects (internally broken belts) (wheel weights that have fallen off) or a bent rim (perhaps from another driver [not you :-) ] hitting a curb) Are the tires worn? Have they been balanced and rotated regularly? You report no vibration in the steering wheel.
Vibrations during braking can be warped rotors or discs (but this doesn't sound like your problem since these kind are usually felt in the steering wheel during braking).
If and only if you feel like you can do this safely,
you can try getting the car up to 55, placing the car in neutral, then switching the "engine off" but leave the "key on" (be warned the power steering will go out but the vehicle can still be steered though the steering wheel will be much stiffer) if you try this (do it perhaps in a vacant parking lot or on an empty road) and the vibration goes away with the engine off but while the vehicle is still moving, then the vibration is engine or transmission related. If this is the case you might hope for maybe a bad engine mount. Bad engine mounts can be diagnosed with the vehicle stopped as well. However, if the vibration remains with the van moving forward but with the engine off, the vibration is likely a problem with the tires or suspension. Try to provide some more info.
If you don't feel comfortable trying the above suggestion, please take the vehicle in to an ASE certified suspension and steering professional for further diagnosis.

Finally, I had a similar situation happen to me today. I was riding with a friend in their van and the vehicle started vibrating more and more around 55 mph. We stopped and carefully checked the front tires (including the lug nuts for tightness) but
found a tire defect. (it had several broken steel belts that caused a bulge in the tire and thus the vibration) We changed the tire and the vibration was gone. Good luck to you.

Check the motor mounts. They might be worn or loose which can cause vibration felt when the engine is reved up or running at higher rpms than at idle.