My pivot bar is held in by a cap

I'm not sure that I understand your window setup. I'm not familiar with any kind of cap that holds a pivot bar in the sash. Can you show me a picture? That may help us identify what your issue is and what we can do to help out.

Ah, I understand now. I was visualizing some kind of peg or clip, but I see now you were talking about the whole bottom portion of the sash frame. Thank you for the photos!

To be honest, it's not common for the pivot bar screws to be obscured by something like this. You usually see this kind of thing on the top of the sash, covering the tilt latches. I can't say for sure, but I would guess that the method of removal of the same: it simply snaps on and off. That said, it can vary from window to window and yours may indeed be held in by something we can't see. 

I'm going to forward this to my colleague who knows more than me about this kind of window. He may be able to offer more insight.

For this type of vinyl sash cap, I would use a thin putty knife and work it under the lip, then follow it down the width of the sash. It should pop off so you can get to the pivot bar. After you replace the pivot bar, you would snap the cap back on.