VGA/HDMI adaptors are not bidirectional, unlike passive DVI-I↔VGA or DVI-D↔HDMI adaptors, because they actually need to convert the signals to a different "format" (HDMI carries only digital video signals; VGA is analogue) rather than simply change the shape of the connector and wire the right pins together. If you're connecting an HDMI output from a graphics card to a VGA monitor, you need an HDMI to VGA converter, i.e. one that converts the digital HDMI signals to analogue VGA ones. Make sure the one you buy is intended for converting in that direction, not the reverse (VGA graphics card to HDMI monitor/TV).