So, as Lee filters were too expensive I bought a NISI filter system a few months ago. See

Now, the filter holder has a circular polariser built in (that you can unscrew and remove, leaving just the holder). They also see a more high polariser as seen here

My question is, if I've got the more expensive polariser am I meant to leave the circular one in? I'm pretty sure this will cause vignetting? If not is it too much? Should I remove the circular one and just use the HD one?

Sorry for the question, I just want to make sure I'm doing this right and getting the correct results.