I'll try to explain though i didn't get all what you said.
Your velocity vector: V{x, y}
Speed: S == sqrt(x*x + y*y)
Normalized velocity vector: N{x/S, y/S}
So you can get a new velocity vector using another speed: V2{x*S2/S, y*S2/S}
Let's assume that you have Speed and X, and want to find Y. Then:
S*S == x*x + y*y
y == sqrt(S*S - x*x)