I bought this adapter from Amazon.com to connect a HP Pavilion 21.5-Inch IPS LED HDMI VGA Monitor to my 2017 iMac, to serve as a second monitor.
While there were other cables/adapters that might have been compatible, I bought the CableCreation USB-C to VGA + USB 3.0 + USB 3.1 Type C Adapter specifically because it listed that it was compatible with the 2017 iMac.
Connecting the VGA cable to the adapter, and the adapter to the iMac, was all straight-forward.
After connecting the adapter to the iMac USB 3 port, the HP Pavilion monitor immediately displayed the iMac desktop.
I did encounter one problem. Though both monitors worked, there was an error message in the top right-hand corner of the iMac similar to:
error – USB Accessories Disabled, Unplug the accessory using too much power to re-enable USB devices
The above may be pretty close to the exact error message, but I did not save the message after conducting an internet search.
After conducting an internet search, I read on an Apple forum that an SMC reset should solve the problem. I then followed the instructions under How to reset the SMC on Mac desktop computers, at the following url.
That got rid of the error message.
I wanted this two-monitor setup primarily so I could use the HP Pavilion monitor to display the Open Broadcaster Software (OBS) application, with a window on the iMac monitor serving as a source.
The CableCreation USB-C to VGA + USB 3.0 + USB 3.1 Type C Adapter allowed me to do this.