Alright, as you know speed is a vector comprising of velocity and direction.
This is incorrect. Velocity is a vector that is defined as the instantaneous change in position w.r.t. time. The magnitude of the velocity is called speed. Speed is a scalar value, and not a vector.
Let's say your object is moving with a magnitude of R in a direction A (in radians). The velocity vector is then given by the ordered pair (R, A) (note that this ordered pair is described using polar coordinates, and not standard Euclidean coordinates). Let (x, y) be the Euclidean representation of this vector.
The relationship between (R, A) and (x, y) is given by the following:
(x, y) = ( R * cos(A), R * sin(A) )
( sqrt( x^2 + y^2), arctan(y / x) ) = (R, A) (note the domain of the arctan function and adjust the angle accordingly).
So if we wanted to increase the magnitude of our velocity vector by a fixed value r, then we would have:
(R + r, A) = ( (R + r) * cos(A), (R + r) * cos(A) ).
Notice that the difference between the new vector and the old vector is simply ( r * cos(A), r * sin(A) ), which is simply the components of the change in magnitude to the appropriate direction.
From this, it is clear that if H is the new magnitude of your velocity with the same direction A, then the components is simply ( H * cos(A), H * sin(A) ).
But as Tepi said, the values are automatically converted for you when you use the built-in variables.