Vibrations are a part of the Royal Enfield package from the very beginning of these bikes. The only way to keep them low is by revving them gently and driving in the highest possible gear at a particular speed. Firstly, the exhaust and other parts should not be altered as they are a significant contributor to vibrations when out of the stock form. Driving between 60-70 kmph on a Classic 350 in the fifth gear would give you the most comfortable ride ever.