First of all, This is a Professional Pilot website, not a computer flightsim wannabe site.
But to answer your question, if the vibration is below a certain level, as depicted by your limitations section of your FCOM and QRH, you can consider the engine to operate normally. But reducing thrust for an instant will not change your speed at all.
If above this limit, you follow the QRH, which might tell you to reduce the thrust until vibration reduces within limits.
It takes several minutes by the way for speed to drop to Driftdown speed at cruising level, with an engine flamout. On B744, around 7 minutes at optimum cruising level. ( weight/fl dependant obviously).