Does it otherwise run ok?? We need more info than this ! Could be fouled plug/burned valve/blown head gasket - choke left on too long.......or a whole raft of ignition problems. You can be SURE it's not the coil........
The idle can't be 'fine' if there is vibration..... Any chance the flywheel is coming loose?? Consider removing the starter - and having a wiggle at the flywheel........