I concur. Flats will take care of the vignetting so next time try to take those as well. As I learned the hard way, flats *NEED* to be taken every time you do an imaging session because reusing flats taken previously may introduce artefacts that are very difficult to get rid of.
Also, The Orion Nebula is a typical example of an object with a very high dynamical range. You took very good care of revealing details in the bright center of the nebula while not loosing them in the faint outer regions. Well done! Having said that, the Trapezium Stars unfortunately are saturated which is pretty hard to avoid. For a next exercise you may want to take a few very short exposure time images to ensure that the Trapezium is not saturated and then try an HDR Composition. This is more advanced stuff of course
Having said all that: you did a fabulous job here! It is a very nice image and much better than my first attempts with PI!!!