HiPS just lets you rehide while in sight. We technically already dropped the requirement for concealment or cover for hiding a long time ago, when we instituted the Tome rules (although I hadn't been keeping up with this diligently, so I let you guys keep worry about sticking in cover or concealment...
). I think if someone is aware of you, it makes sense that you'd need Cover or Concealment to hide, but I'm just going to stick with the Tome rules officially.
So yes, as long as you don't reveal yourself, you can remain hidden near the Vrock w/o HiPS, even without cover or concealment. If you are revealed, you won't be able to re-hide w/o HiPS, unless you get out of LoS.
I should probably also point out that Marlowe was walking
towards the rubble that the Vrock is on when he walked into the dead magic zone. It's kind of hard to note that on a map, especially one as crappy as mine.