This is at best a partial answer, but I believe I have collected some pertinent information. I'm the author of that other question you link to, and my goal is exactly the same as yours: use the Nexus 5 with a projector with VGA input only.
You distinguish between VGA adapter and converter, which is part of what I was asking for in my question, and from the answer I got I thought there is no such distinction. Especially after I noticed that this HDMI-to-VGA product, which in the description claims not to need an extra power supply, actually does have a Micro-USB port to get power, and many customers comment that it only works with extra power, I assumed that the difference is a matter of marketing. In the (German) product description it says at some point "Adapter" and at another point "Konverterkabel" (converter cable). I still don't know whether there is such a difference. If there is, I'd assume that an "adapter" merely makes an analog signal which is somehow hidden (?) in the HDMI available, while a "converter" actually contains D/A circuitry.
I had a look at the "SlimPort to VGA adapter" from Analogix but didn't buy it for the same reason you give; moreover, the description mentions "LG G2" and "LG Optimus G Pro", but not the Nexus 5. The Analogix website also shows an "HDMI/VGA Combo Adapter", which appears not be for sale yet. I sent an Email to Analogix requesting clarification, but they did not reply.
In the end I decided to buy the "HDMI Adapter" from Analogix and the HDMI-to-VGA thingy linked to above. The first works perfectly, but the second doesn't work at all, at least not in combination with the first. I tested it using a projector and two different monitors, and in all cases there wasn't even a sign that a signal was received – with or without extra power. I'll send it back, but I don't know which other product to try next.
A possible explanation why it doesn't work would be that the native resolution of the Nexus 5 of 1920 x 1080 (Full HD) is too high for a VGA connection, or for the respective device. Since the Nexus 5 has no HDMI output settings, I used the app "Screen Settings" (needs root!) to change the resolution. This appears to work on the device screen (apparently scaled up again to the screen's size), but doesn't change the fact that VGA output isn't recognized. Strangely, if I connect to a monitor via HDMI, the monitor still reports an 1080p input signal. This might be a quirk of my monitor, but it might also indicate that the changed resolution only affects the software side but not the signal. I'm not sure whether HDMI supports arbitrary resolutions, but I think there should be at least 1080p and 720p.
Another possible explanation is that the HDMI output is "protected" by HDCP, and a "legal" HDMI-to-VGA converter won't simply strip the HDCP, thus just disabling output for an encrypted input. According to this thread on xda-developers, HDCP is activated by default and for all content on the Nexus 10, and the only way around it is to install a custom ROM. I suppose the same holds for the Nexus 5.
I made a post on Google's product forums about this, the chances for getting an answer might be slightly larger if more people favorite this and/or chime in.