You can use the Calculator filter for that. Try those formulas:
v_r=(x*v_x+y*v_y)/sqrt(x^2+y^2)
v_t=(x*v_y-y*v_x)/sqrt(x^2+y^2)
Of course, x and y are coordinates. They can be accessed from the Calculator.
And please upvote this feature request if you’d like to have a built-in filter that can do such a transformation: Transforming results to user-defined (local) coordinate systems