I have the 1rst gen Chromecast, and it's perfectly working on a normal (1080p) TV, but I can't find out any tutorial or anything to set the video output resolution from 1080p to 1050p or lower, to go on another display. I think the Chromecast think it's a 1080p screen because I'm using a HDMI to VGA adaptater. Can somebody help me please? I've tried all I can, but found nothing...
You can't set the resolution.
The Chromecast will always output the best resolution that it can based on the source file, your network, your TV, and the formats that that Chromecast model supports.
If you are not getting a good picture, it's possible that the converter you are using is confusing the Chromecast. Google does not officially support the use of converters. Check with the maker of your converter to see if they have adjustments.