each riser & gpu combination should be connected to same PSU. you can use one VGA cable for the GPU and the riser connected to that GPU. or you could use one VGA cable to power multiple risers, but it would be easier just to use one VGA cable per gpu/riser combination. you probably need the splitters linked to on ebay.

if you're keeping your GPUs under 120 watts (like around 90-120 watts) you could put 2 GPUs on each VGA cable. I think those VGA cables are rated for 300 watts, running 225 watts on one VGA cable would be no problem.

I have a rig right now where I have 2 GPUs per cable and I keep each card under 110 watts. no issues