Looks promising, though I thought it was gospel not to use SATA power because of the cards drawing more power than it's rated to handle?
I have connected both Biostar SATA connectors directly to one PSU like is recomanded on Biostar site: http://www.biostar.com.tw/app/en/event/crypto_mining/page4.htm. Also the risers and the GPU are connected to PSU with other two cables. I don't think it's something wrong... I hope

Do not worry, 8pin can deliver 150W, so for vega you have a power 300W from 2x8pins and vega powerdraw should be around 220W.
I heard that for sata is max somewhere around 25W... But this depends on quality of cables and connectors...