BOSTONUsing his unparalleled ability to nod after his superiors speak, Thomas J. Mieritz, 39, rose to the level of vice-president at Fidelity Investments Monday. 'I knew Mieritz was the man for the job the instant I started talking. He was ready to get on board with every one of my proposed mutual-fund investment initiatives,' Fidelity chairman Edward C. Johnson III said. 'I thought, 'Now, there's a man who makes smart decisions without a lot of hullabaloo.'' Johnson added that, if Mieritz can master boot-licking, buck-passing, and myopic self-satisfaction, he'll probably run the company one day. 