Write your dopplar equation twice, once with positive Vo and again with negative Vo. One gives the maximum frequency observed, the other the minimum, so the difference between the two is your 1.83 Hz. That's your clue to subtract the two equations. I think you will be able to get Vo out of that.
Regarding the amplitude, I wonder if you have an equation something like
x = A*sin(ωt) for the position as a function of time. And can differentiate it with respect to time to get a similar equation for the velocity. The two of them would constitute a relationship between the amplitude and the maximum velocity.