You downloaded an RGB video without an Alpha channel. Go back to the download source and find the version labeled "Apple ProRes 4444" or "Animation Codec." If you cannot get that, use the "Color Key" option in the Stinger settings to key out a solid color background (usually black or green).
A vMix stinger is an animated transition that masks a scene switch with a full-screen graphic, typically used in high-end sports broadcasts and professional live streams. While standard fades are functional, a stinger provides a branded, high-energy bridge between camera angles or segments. Top Sources for vMix Stinger Downloads vmix stinger download