You can read official vMix API documentation here: vmix.com/help25/index.htm?DeveloperAPI.html

How to Use

You won't be able to access the vMix API over HTTPS, so you'll need to download the source code from GitHub. You can either clone the repository if you have Git installed, or click Code → Download ZIP. After this go to the vmix-master/ folder and open the index.html file in the Chrome browser.