A bit late on this but it seems the 533 has a similar issue to the 294. ZWO are correct do not use bias frames as it seems the bias is not stable on the sensor (if the same issue as 294), use Flats and Dark Flats, and be very careful to match them, plus Dark frames. On the 294 it is suggested using low offset, 4 to 14.
Vignetting is not a particular problem of the RASA. This is my first attempt at the Iris, https://www.astrobin.com/plzzrw/?nc=user
Preserving details in difficult Iris
Re: Preserving details in difficult Iris
Another point. I think you said up thread that your flat exposures were very short (0.1 sec or less?). I know if I do this with my 294, the flats are problematic. For the 294 flats must be greater than 3 secs - which means you need a very dim but even light source. I don't know if the 533 is affected by the same problem, but it would be worth trying longer flat exposures. For the 294 the advice is very definitely to use dark flats, NOT bias frames.
Good luck
Paul
Good luck
Paul