However, the hosts grew into the encounter as time elapsed and saw their improvement rewarded after James Tavernier won a penalty on 13 minutes. Agard stepped up and kept his composure to fire beyond goalkeeper David Button from 12 yards.
Having won the penalty, Tavernier came within millimeters of playing a bigger role in United's second after his 35th-minute free kick cannoned off the post.
Nevertheless, Agard was on hand to double the advantage shortly before the interval, capping off a free-flowing move with a clever finish to give Steve Evans's men a two-goal lead.
The 24-year-old should have collected his hat-trick on the hour mark but headed a clever Nicky Adams cross over the bar from six yards.
However, Vuckic struck in the dying embers to cap an impressive victory for the Millers.