My gpu dont support VGA but motherboard video does. Can i hook it on the motherboard and will that make the pc use only the onboard gpu.
Yes, if you connect your monitor to onboard iGPU, everything displayed on that monitor will be generated by onboard iGPU only, and in that case your GPU will do nothing.
Integrated GPU usually gets disabled, if discrete GPU is present in the system.
Simultaneous operation of iGPU and discrete GPU can be enabled in BIOS, but you'll have to remove discrete GPU first.