I have captured footage perfectly, and rendered it, from the cameras electronic
16:9 aspect ratio. Now, if I put an anamorphic adapter on the lens instead, to use
all of the pixels, will Vegas unsqueeze it automatically, or do I have to tell it to?
I'm not sure exactly what you're asking, but you should do your project and digitize your media as if it were normal 16:9, just like doing it from the built-in mode of your camera.
I know it's been discussed before. A few months ago...
I'd try a search with "widescreen" or"16:9" or "16x9" or "anamorphic" keywords.
You'll find it for sure... hope that helps !