wide angle vignette showing, but not during recording
 

I'm using an FX7 that came with a Merkury Optics wide angle lens.
During filming, it shows fine on the camera's LCD (no vignette). But when I captured the footage, there is a large vignette. Also, the zoomed shots do not show the vignette on the captured footage.

How can I get around the vignette problem?
Is it because this is a "cheap" lens?

You don't see it on the camera screen because the full frame isn't shown (the HVR series cameras have an option called "allscan" which shows the full frame, the HDR series do not). Yes, this is pretty typical for cheap lenses, or lenses which are not properly matched to your camera.

Sounds like you already found the solution... zoom in a little. Otherwise, you can englarge the image a bit in your editing software. But before doing any of this, I would take a critical look at footage with and without the wide adaptor. You may find that this lens creates other problems like chromatic abberation (rainbow colors on the edge of objects), barrel distortion (warping of the image such that the horizon looks like a "smile") or soft focus.

I thought about re-sizing the image in post also. But yes, I do get a lot of soft focus, or extra glow on my subjects.

Thanks for the information.

Cheap glass... 'nuf said. Sony makes a WA for this cam, and it's not cheap, nor terribly wide (.8x), but it doesn't degrade image quality or vignette.

You definitely get what you pay for when you buy inexpensive import "aftermarket" lenses - HD shows all the flaws, and while it's cheap for the manufacturers to add the letters "HD" on the box, it does not a "HD" lens make...