General Motors--a "cover-up" [Note: The Vibrating could begin at 30 M.P.H.]

Salespeople had previously driven the vehicle (Buick Riviera). So, it could not be considered a Lemon--But I walked away from it / gave it back. Sadly, my father was driving the family back from (NC to VA) a road trip we had taken, (in a different GM product, we rented) when the same type of vibration occurred. So, he and I both, simultaneously looked at each other. I'm curious if anyone else has experienced this (vibrating).BTW, a Company Representative came out, we drove the Riviera it did the vibrating thing but, because I was going over the Interstate speed limit of 55 MPH, the GM Rep. dismissed the whole idea that there was a problem(without the new tires and alignment, vibration was intermittent but generally would start at around 30 MPH).