so i've zeroed-in at about 12 yards (as far as my basement will let me), and plenty for my backyard, for now.
Vixen Viper PST 1-6x comes to focus without issue, at 12yds.
As for parallax, I don't see an issue. on a 1-6x scope set to 1x or 2x, you can't really be expecting any pinpoint accuracy, especially beyond 10yds. So at 1-2x, although you'll "drag" the crosshairs a little, by moving your eye around the exit pupil, that margin for error is larger than anything the parallax would contribute, in my opinion/estimation.
At magnification: I zeroed the scope in at 4x, since that's what I'm likely to be using most often. That yields a 6mm exit pupil (24mm objective / 4x mag = 6mm EP)*. 6mm exit pupil leaves very little wiggle room in the eyepiece to be off-axis. However while target shooting at 12yds, it is "technically" possible to be on the wrong side of parallax issues (wandering crosshair). But my take on that is: Nobody holding an air rifle who knows what parallax is would ever take such a shot. It's just a wasted/bad shot.
So my take on parallax is: The Vixen Viper PST has no practical, real-world issues with parallax at 12 yards.
*A note about exit pupil numbers, above: the EP numbers I mention above are assumed, not measured. Reviews put this scope at 24mm exit pupil at 1x mag, so I'm going to assume there is no clipping/obstruction within the optic. Vixen also claims 24mm, and knowing Vixen by their reputation through binoculars, they are unlikely to be misrepresenting their claimed specs.