Typically in vMix you use the Multi View tab in the settings for a particular input, in order to create a layered combination of different inputs, all within a single input module. However, since you can't add a layer behind the native "base" layer, you can't add a video behind a title in the Title input. But...

1) You could create a Virtual Input from the video you want as the background, and then add the required Title as a MultiView layer in the virtual input, in front of the video. Or, if you don't want the video full screen, but us a window combined with the title...

2) Create a "blank" (colour black) input. Go to the Colour Key tab and move the Luma Key slider to the right a bit, which will make the base layer transparent. (There's probably a better way to do this, like importing a transparent PNG or BMP file, rather than using the Luma Key, but this method will demonstrate the principles of how it works). Then go to the MultiView tab and add your background video, which can be scaled/positions as required. Add your Title as a higher number layer (or even add multiple title layers), and you then have a "composite title" input which can be used as an overlay just like the original title input.