I am unclear why vinculin is not showing a difference if you load twice as much. Perhaps you have reached saturation on your detection in both lanes on exposure to ECL and so you no longer see a difference

For our high molecular weight proteins (>500 kDa) we routinely use either beta-actin or GAPDH. Even if we run a 3-8% Tris Acetate gel for these HMW proteins - beta actin will still be on the gel. This may be a better control for you. I would try a few controls.

Perhaps you can also do a serial 2-fold dilution with your cell lysate of choice across 6-8 lanes and then see which control shows the best linear decrease across this series.

Good luck.

LRP-1 would be a great protein to work with. it is around 600 Kda..2 fragments one is 85 and the other 515 kDa

According to abcam vinculin is a loading control

http://www.abcam.com/primary-antibodies/loading-control-guide