The reigning champions were looking unstoppable after taking 22 points from a possible 24 in their first eight games, but were second best against Ilves on Wednesday, losing 3-0.
Their attempt to bounce back at home, however, took a blow midway through the first half when a curling cross from Prosper Ahiabu was nodded home by Nicolas Fleuriau to put VPS in front.
Midway through the second half, Fleuriau was replaced by Olli Jakonen, whose first involvement was to chase a cleared KuPS free-kick, and his determination was rewarded when a sloppy back pass allowed him to flick the ball past stranded goalkeeper Johannes Kreidl and race away to score into an empty net.
The win takes the Vaasa side to four matches unbeaten, with the three victories all coming against sides in the top half.
KuPS' lead at the top is now four points after Inter Turku recorded a 1-0 win away at SJK and, worryingly for the Canaries, unbeaten Inter have two games in hand.
The only goal of Sunday evening's match came in the 11th minute, when the visitors worked the ball across the edge of the box before Loic Essomba squared to Luka Kuittinen, who had time to turn and curl the ball into the bottom corner.
The Turku side's record this season now reads five wins and three draws, while early pacesetters SJK continue to slide down the table, dropping to sixth after taking one point from their last five matches.
Storming past SJK in the opposite direction are HJK, whose 4-0 win at Mariehamn is their fourth victory in a row, and has taken them to third in the standings.
An HJK win means a goal for Teemu Pukki, and this game was no different, as the striker broke the deadlock after just over half an hour, latching onto Pyry Mentu through ball to score his fifth of the league season.
In the second half, goals from Alexander Ring, Santeri Hostikka and Giorgos Antzoulas ensured not only a biggest win of the season so far for HJK, but the third match in a row in which they have scored four.
Mariehamn sit eighth on 10 points, and continue their run of always following a victory with a loss this season.
Elsewhere, a Kerfala Cissoko goal in the first minute for the hosts was cancelled out by Didrik Hafstad late in the first half as Jaro were held 1-1 at home to Gnistan.
However there were no goals on Friday night between KTP and Haka, while bottom-of-the-table AC Oulu pulled off a small shock by drawing 0-0 away at Ilves, who slip to fourth.
Oulu stay bottom on four points, one behind KTP and four shy of Haka and Jaro.
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