Not all cards have a DVI-I connector, offering only DVI-D connectors, so you are wrong - a dvi/vga dummy plug will not work.  R9 290s do not work with those dummy plugs, and frankly, those 30sec dummy plugs are now pointless and stupid (modern drivers don't need them)

With 290s, some mobo's won't post without a monitor attached. Easiest solutions are attach a monitor, attach an hdmi switch, and buy a DP to VGA adapter (with active chip) and make a dummy plug with that.