To actually record the x,y coordinates, you have to modify the <data> element. The VAS script only records a percentage score adjusted for different screen resolutions, etc.. If you use a shape or a picture as your scale doesn't make any difference. Consider this simplified sample code:
<values>
/ x = 0
/ y = 0
</values>
<trial mytrial>
/ stimulusframes = [1=blackblock]
/ inputdevice = mouse
/ validresponse = (blackblock)
/ ontrialend = [values.x=trial.mytrial.responsex;values.y=trial.mytrial.responsey]
</trial>
<shape blackblock>
/ shape = rectangle
/ color = (black)
/ size = (400px,100px)
</shape>
<block myblock>
/ trials = [1-10=mytrial]
</block>
<data>
/ columns = [date,time,subject,trialnum,trialcode,latency,response,values.x,values.y]
</data>
Regards,
~Dave