Dundee United handed a debut to goalkeeper Marc McCallum as they made a raft of changes from the side that drew at Inverness, while Dons manager Derek McInnes gave Vernon his first start since January.
An action-packed first half saw the home side have an early penalty shout when Russell Anderson tangled with Ryan Dow. The referee was unmoved and moments later the visitors took a seventh minute lead.
A long clearance from Jamie Langfield was won in the air by Vernon who linked well with Peter Pawlett before firing past McCallum from close-range.
The home side almost got an equaliser four minutes later when Nadir Ciftci stung McCallum's palms with a shot from the edge of the area.
Aberdeen should have extended their lead midway through the half but Barry Robson could only head Niall McGinn's cross wide.
And they were punished in the 34th minute when Sean Dillon's effort took a deflection off Reynolds and past the stranded Langfield.
Aberdeen had Langfield to thank for going into the break level after two stunning one-handed saves. First, the 34-year-old diverted Robertson's volley wide before clawing a Mark Wilson's effort onto the crossbar.
United were relieved not to have gone behind early in the second half when McGinn's duiving header was ruled out for offside after McCallum spilled a Cammy Smith shot.
McCallum then made a point-blank save from Vernon, diverting the ball behind. But the 21-year-old misjudged the subsequent corner from Robson allowing Vernon to steal in at the back post and head the visitors back in front after 55 minutes.
Ciftci had a chance to equalise for the home side moments later but blazed his effort over and on the hour mark Dow had a goal ruled out due to a marginal off-side decision.
With 20 minutes left they should have got back on level terms but once again Langfield was on hand - this time to deny Brian Graham from close-range.
Vernon sealed his hat-trick after 76 minutes after a miscued header back from Gavin Gunning allowed the 30-year-old to sneak in and round McCallum before slotting home.
Gunning was lucky to stay on the field after aiming an elbow at Aberdeen substitute Calvin Zola in injury-time, but it made no difference to the outcome as the away side edged closer to a second-place finish.