Vehicle Shudder problems of the 2014 Hyundai Veloster

Sold a 2014 Hyundai Veloster with grinding brakes, acceleration problem, and vibration issues. I was told everything would be fixed. I returned the car within 24hrs. The car vibrates dangerously at 50mph and above. The car hesitates when taking off, trying to pass, or get on freeway. This is also dangerous because cars already going 70mph could of hit us while trying to get on freeway. Sometimes while giving the car gas it would just rev up the rpms without doing anything. This also scared me into returning the car. I went back to test drive the car to see if problems were fixed, and the mi gente auto dealership told me they just replaced tires and that fixed the problem. I told them they can keep the car, because I don't want to keep coming back for same issues. I was threatened with repossession will be added to my credit.