SetViewPort Problem

Because I have A LOT OF SetViewport / DrawImage Commands in my game source I tought of a possibilty To overwrite those two Blitzfunctions, but I have some problems with the math how To calculate the news coords.

The source should look something like this

SuperStrict

Graphics 640,480,0

'Viewport coords
Global vx:Float, vy:Float, vw:Float, vh:Float

Local t:TImage = LoadImage ("gfx/mainmenu.png")

While(1)
Cls
SetViewport 200,200,100,100
DrawImage t,20,20
Flip
Wend

Function SetViewport( x:Float, y:Float, width:Float, height:Float ) 
	vx = x
	vy = y
	vw = width
	vh = height
End Function

Function DrawImage(image:TImage, x:Float, y:Float, frame:Int=0)
	
        'Calculate new coords, I don't know how to calculate them

	Local rx:Float = vx - x
	Local ry:Float = vy - y
	Local rw:Float = vw
	Local rh:Float = vh

        'Recalculate x/y coord

       x = ???
       y = ???
		
	ccDrawImageArea(image,x,y,rx,ry,rw,rh,frame)
		
End Function

I know that other commands (like DrawRect etc...) won't use the viewport, but that doesn't matter for my case...