On-board video card automatically gets disabled if you install a PCIe graphics card. I am guessing that your new graphcs card does not have vga port. DVI to VGA adapters are cheap so you might wanna grab one to run your VGA monitor.
But if your new graphics card is on x1 PCIe, you can run both graphics card at the same time. Just enable the on-board card in BIOS. The right-most monitor on my sig below is on VGA port via on-board video card.
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