Wafula stooped to head home Ashley Hemmings' pinpoint delivery from the left as the Pilgrims snatched all three points against their 10-man visitors.
Dan Bradley's header had put Harriers ahead, only for Jordan Keane and Ben Middleton to turn the tables in United's favour before half-time but substitute George Waring levelled moments after Lee Vaughan had been red-carded for a rash challenge on Hemmings.