Peter, your NVGs are of relatively cheap design and the optics are probably very basic - this is what is causing the edge distortion, just like a cheap camera lens. The visual acuity on a Mil standard set of goggles doesn't fall off at all across the field of view - but it does take some getting used to having a 40 degree FoV. The technique with goggles is to scan by moving your head (this is one reason neck pain can be associated with extended NVG ops) so you look directly at your object of interest.
The lack of peripheral vison using NVG, coupled with the lack of depth perception is the main reason proper training and currency are required.
The many shades of green can make an object that is very familiar in daylight, completely unrecognisable under NVG and makes pure NVG map reading a bit of an aquired skill. 'Fly to the green field on the nose'
PS don't use them at all in daylight as you will damage the tubes (too many photons freeing too many electrons from the tubes).