Assuming that:

• Your vector2 are the (x,y) coordinates of some point

• Your reduction factor is a scaling reduction (e.g. I want to decrease my size by 20%)

• The origin is at (0,0) and at the center

This becomes a pretty easy math problem. Direction isn't really relevant in that case.

public float reductionFactor = -20;
public Vector2 origin;

void Update()
{
    Vector2 target = GetIntersectionWithCameraBounds(follow);
    // scale the percentage to 0-1 and then look for our target size
    // (e.g. a 20% reduction means we want to be 0.8 of our original)
    float targetRatio = (100.0 - reductionFactor) / 100.0; 

    Vector2 reducedTarget = new Vector2(target.X * targetRatio, target.Y * targetRatio);
}

If you are using Unity or some framework, there may be built in ways to handle that type of transformation.