I'd like to know which cable adapter you have that claims to be "passive". From what I know (check http://www.tomshardware.com/answers/id-2596178/difference-passive-active.html for a good summary, especially the last post), there are 2 "definitions" of active vs passive:
- Passive = no external power source required, i.e. all powered from the power supply of the device that contains the source signal.
- Passive = no transformation of the signal, i.e. the signal is just rearranged from one input pin to another output pin.
From that definition, the manufacturer of your HDMI-VGA adapter probably used definition #1 as the "passive" description. However, given the fact that HDMI is a digital standard, and VGA is analog, there is no way the adapter could also be "passive" from the perspective of definition #2.
Now to your question's answer: I use a "passive"(definition #1) HDMI-VGA adapter regularly with my rPi's (both B, B+ and 2B) and it works, IF AND ONLY IF I plug in the adapter before the rPi turns on AND the monitor I connect to is already turned on as well. If I try to add my HDMI-VGA adapter while the rPi is running, I get no signal. Can you try this?
If that fails, make sure you have looked at all the HDMI configuration options in /boot/config.txt - perhaps you need to force something there to make the dongle work.
I would NOT start with trying to modify the dongle - it should be properly powered from the rPi's HDMI port ("passive" as per your manufacturer) - the cause is most likely in the two things I mentioned above.