Ryan Christie appeared to have earned a point for Inverness with a 74th-minute leveller at the Tulloch Caledonian Stadium on Wednesday.
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But there was still time for Stuart McCall's Motherwell to clinch the maximum haul, which saw the visitors leapfrog Aberdeen, though they have played a game more.
It took until the 40th minute for Motherwell to make the breakthrough, Fraser Kerr sending Henri Anier through on goal and the striker took a great first touch before calmly slotting beyond Ryan Esson, the ball hitting the post before nestling in the net.
Inverness had failed to trouble the visitors' defence in the first half but went close to levelling when Greg Tansey's venomous effort rebounded off the post.
It took a peach of a strike from Christie to level the scores with just over quarter-of-an-hour remaining, the teenager drilling past Gunnar Neilson when slightly off balance.
Motherwell were not to be denied, however, and Inverness were caught out at the back, as Craig Moore and Vigurs raced clear of the defence, the former slipping a pass to the latter who in turn tucked the ball away neatly.
The win means Motherwell are well in contention to secure a UEFA Europa League berth and are six points clear of fourth-placed Dundee United, while Inverness' European hopes were dealt a potentially fatal blow and they are now 10 points adrift of Aberdeen.