use the VGA cable to power riser & GPU.
Is it ok if I use one VGA cable to power a gpu and the riser for another gpu all on the same cable?
That depends on the cards you are using,a colleague of mine used to connect two Gpu-s within the same Sata cable which can only support one Gpu and the result was that a Thermaltake PSU blowed away,we were using two of them for a six Gpu rig.Better stick to the plan it works best,do not overload your cabling and you should be fine.