Is it a reference to Windows Vista? Is it an actual vista of an entire level from top? Where can I find this particular secret?
As usual in Serious Sam 3, the area in question is in serious plain sight. Acute players will have noticed this small green blinking item in The Power of The Underworld, roughly 400 kills into this serious level (after the serious infinite rocket storage).
It's seriously right under the crosshair. But how the hell would you get up there?
Climb on this seriously damaged wall further ahead, opposite the construction area leading to the next serious part of the map.
From there you'll see serious "secret keys" blinking in serious purple. Go back to the where that green item is located - the serious gates will now open for you.
You'll find a seriously long ladder on your left, before heading through the second gate. Climb it up, and, sure enough...
It's no crash to desktop, but it's a serious sight to behold, plus you get to have a peek on the following serious area. A very good peek I might add, since the green blinking item just around the corner is a serious sniper rifle. Unfortunately, it only comes with 5 serious shots and it appears it only can hold 10 anyway.
Remember to take the ladders on your way down, or you'll simply crash under your serious weight and suffer a seriously humiliating death. You can simply start taking the stairs with E, then jump off them and press E just before reaching the ground serious fast.
It is in the Power to the Underworld level.
From this walkthrough:
In the final courtyard before exiting into the final open arena, on the middle left side, on the roof of the rooms there, there are some Secret Keys sitting against the wall. Simply climb up towards the final open arena, but instead walk along the edge of the wall until you find them. After getting the keys, go back to the pylon with the locked metal-gates, in the previous wide corridors. The gates will open revealing a ladder leading up to a secret Vista.
Here is a video that shows you where to go: