I have a Dell OptiPlex 5050, and it supports 3 displays. I have already determined that all the displays work through both display ports, so all three will detect. However when i connect the monitor to HDMI to VGA it will not detect. My monitor has only display port and VGA, so i had to get a VGA to HDMI cord. I want to know how to get this working, it would tremendously help me with work.
The use of adapters, like your hdmi to vga cable, is not supported.
That said, try a different (or better) adapter. For example, I am using this one: http://www.microcenter.com/product/485100/hdmi-male-to-vga-female-adapter-w--audio-support---black Inland HDMI Male to VGA Female Adapter w/ - Micro Center
The above link is provided for your convenience.