Why doesn't the projector find the Macbook Pro when connecting a VGA cable to the HDMI port on Macbook Pro via an adapter? It does work on a Windows machine. I know that to fix this problem one needs to connect the VGA to the Mini Display port (via an adapter of course). So is there another way around this? Or do we have to connect to the Mini Display port.
After a few months of experimentation, we found no 3rd party (i.e., non-Apple branded) HDMI <> VGA adaptors which worked well and some which didn't work at all on our Macs. We bit the financial bullet and purchased Apple-branded adaptors and they work perfectly.
We found the same with Thunderbolt & USB <> gig-Ethernet adaptors and Lightning <> HDMI adaptors, too. Remember, you may save $$ purchasing 3rd-party adaptors, but there are tradeoffs in either operation or quality, frequently both.
Most 3rd party adaptors have little or no strain or twist relief in the cable attachment inside the adaptor. We had slews of non-Apple Thunderbolt <> VGA adaptors die after only a week or two of use because the cables literally twisted out of the adaptor body.
Being that VGA is an analog signal, and (Mini) DP / HDMI are both digital signals, an adapter will be required.
Given that you wanted a solution that used HDMI, a powered HDMI port is necessary, which is not be included with the MBP, in addition Apple does not support such a configuration:
Can the HDMI port drive analog displays (VGA displays, for example) using adapters? No. The HDMI port will only drive digital connections (such as DVI) using the adapter that came with the Mac. The HDMI port cannot be used for analog displays such as VGA. You can purchase a Mini DisplayPort to VGA adapter separately to drive analog displays.
I would think you need to do with the Mini DP converter, if VGA output is necessary. Alternatively, it is possible to get a more expensive powered active converter.
On my 2014 MacBook Pro with 2 Thunderbolt ports I found HDMI port to HDMI-VGA adapter did not work, but if I used my Thunderbolt to HDMI adapter then plugged the HDMI to VGA adapter into that, the VGA signal to the external monitor worked fine. Neither adapter are Apple branded, just generic.
I also had to turn off Dynamic Switching of the graphics cards (I use gfxCard Status 2.3) and use Discrete Only to prevent flickering on and off of both monitors. It still happens occasionally, but it's bearable.
EDIT: After a while the Macbook Pro intermittently stopped powering the external display, leading to both displays flickering on and off more often. Maybe I will try an Apple adapter or a different monitor with HDMI input.
I also have an HDMI to VGA adapter and it doesn't work on my Mac, it does on my Raspberry Pi, haven't tried a PC yet.
There is a high probability that its Apple's System Integrity Protection that's stopping 3rd party adapters from working.
This can be disabled, see How to Disable System Integrity Protection (rootless) in Mac OS X.
I was attempting to get a USB to Ethernet adapter working on my MacBook Pro 2015 running El Capitan and it wouldn't work. I disabled this feature and added the driver for the adapter and it worked. See How to use a Generic USB 2.0 10/100M Ethernet Adaptor RD9700 on Mac OS 10.11 El Capitan.
There was a down side though, after this was installed the adapter caused my mac to crash twice. It was causing Kernel panics and wouldn't start hence the name System Integrity Protection.
Since I've restored without it I haven't had problems, it could of been the driver so you could just disable System Integrity Protection and see if it works.
I can confirm the answer of user3735534 works.
Here's what happened in my case. I used to have a Toshiba laptop with VGA port to connect to my VGA monitor. When I upgraded to Mac Book Pro, I gave away the Toshiba but not the VGA monitor.
I purchased an HDMI to VGA adaptor. The brand is Sandberg. It's a small square plastic box which has the HDMI cable protruding on one end and the VGA slot on the other end. This adaptor also has a small slot on one side for the micro-usb.
After connecting the HDMI and VGA, Mac Book Pro detected a second display (in settings, I could choose to mirror or extend the display). But the VGA monitor stayed black - no display at all. I used the micro-usb port of the adaptor and plugged the USB end to my Mac Book Pro. The VGA monitor then came to life. Now I'm using it as an extended display.