Not only is there a front face and rear face to the flex disk, three of the flanges are designed to attach only to the transmission while the other three attach only to the driveshaft. Nuts, bolts and washers can only go one way or the new disk will be flexing in ways not intended, and yes, you can get vibrations from it. Is the mechanic that installed it an experienced MB mechanic? Did he use new bolts? If there is any play between the sleeves and the bolts, the bolts must be replaced. Also, I always replace the self locking nuts.