NEWBIE Color vignetting and weird pattern

The wierd pattern is called walking noise and it happens when you have a slow drift in one direction. This is bad because noise reduction won't eliminate it completely. The only way to get rid of it is to do something called dithering. Every few frames you move the telescope randomly by a few pixels. Since you are running unguided the best way to do it is to use NINA and dither using the "On Mount" guider option.

On flats, just take sky flats as it's super easy. Just point your setup toward the zenith and shoot a series of frames with exactly the same orientation and filters as you used at night. No flat panel or t-shirt needed, easy!