How do I calculate relative velocity?

Based on a single Y rotation (Rot) of an object on the XZ plane that periodically updates its position (Pos) by adding its velocity (Vol), how can I calculate relative velocity?

So I.e. Using Unity's standard vector libraries

Vector2 Pos = Vector2.zero; //(0,0)

Vector2 Vol = Vector2.one; //(1,1)

float Rot = 30; //30 Euler degrees

void Update()

{

Vector2 RelativeVelocity = MethodForCalculations (Vol); //Calculates based on velocity and angle

Pos += RelativeVelocity; //Changes position based on velocity and angle

}