You are correct that vMix only support two stingers. These are loaded into memory to speed up their use. Some users however have been able to create additional stingers by using a combination of video inputs and triggers to extend the number available. I am sure that someone will explain how this can be achieved. However a quick, if not perfect way to use additional stingers is to change the input. To do this click on the Gear icon on the bottom right corner of the stinger input. At the top right of the General Screen click on change. This will take you to the Stinger selection screen and here you can browse for the stinger you want. vMix will show a thumbnail of previously used stinger. Select the one you want and click OK. It will now load the stinger into memory and you are good to go. This will take a little bit of time, but it is workable.

You will also need to become familiar with the settings for the stingers. These can be found by clicking on the overlay button, bottom right of screen, and changing the overlay dropdown to stinger 1 or 2.