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Field curvature will be minimised by a field flattener. If you can get one specific for your scope, great, but a "generic" flattener will do.
Vignetting can be combated by getting a large enough focuser so that the light path to the ff CCD isn't hampered by anything internally. You'll need at least a 2" focuser or larger for a 35mm CCD chip. Same goes for the flatteners and anything else you connect up in the imaging train. Anything can cause vignetting, but flatteners can be a source of the problem. Sometimes, it's unavoidable, so you have to live with it and just adjust your piccies accordingly.
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