Vaal side area spawn chance?

I'm just missing one for the challenge, which should spawn in the Foothills (Curator Mien), but after doing that area over and over again now for quite a while, I'm yet to see a Vaal side area spawn there. What are the chances for it?

Do they spawn in the same location each time, if they spawn, and where is that? That way I can go right for it and check. Would at least save some time.

I've gained two levels since it's a fairly high area, so it's not all bad, but yeah...

No idea how the odds are. I suggest you just play and do the side area bosses whenever you get the prophecy for vaal side area. It´s a fairly common one.