Vegas does have 'multi-layering' as you call it, it might be better to call it video tracks rather than layers as it isn't an image editor. I don't use Vegas myself but you can certainly add more video tracks (where you stack either video or still images on top of one another) and audio tracks in both Vegas and Adobe Premiere, chromakeying and green screen are also included in Vegas. Download a 30 day trial of it and try it out.

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