I've heard people complaining about changing tire after only 20~30K miles on BMW original tires. You need to find out the tire type and the mileage rating (each tire may wear out differently). Suggest you go bimmerfest forum (can google it) where you may find more info (such as engine mount issue). But for vibration, it could be something else(but not 100% sure). BTW, $400 for 4 BMW tires sound cheap. Make sure they are not re-treaded ones.