Chris, You can improve vignetting in photoshop. I had a go and got this. Here's what I did :-
Open the image. Make a copy of it as a layer. Select this layer and pick filter/noise/median from the menus. Set radius to max and click ok. You now have a blurry image which is mostly the gradients. Now use the clone stamp tool to blot out any brightness from the target (M20), or any bright stars etc. Now select filter/blur/Gaussian and turn up the pixel size to smooth it out. Now blend this blurry layer as difference and adjust opacity to suit.
This works most of the time and a bit of trial and error gets you used to it, but flats really are a must have to start with
