Hampshire 199 for 6 (Vince 65, McDermott 60, Crocombe 3-31) Sussex 177 for 6 (Seifert 100*) by 22 runs.
Hawks had lost their first four matches but jump off the bottom of the South Group, while Sussex remain fourth after three wins and three defeats.
A switch appeared to have flicked with Hampshire after their poor start to the tournament, with aggression channeled into ground strikes rather than chasing sixes. Vince showcased this by beautifully caressing the first ball of the match through the covers, having been put in.
It was just one of a range of classic Vince strokeplay which gave the hosts a rocket of a start. At the other end, McDermott got off the mark with a huge straight six with his more brutal style. That was the first of five maximums from the Australian, with a scoop the outlier from straight and midwicket hits.
Vince, who moved up the order having batted at No. 3 previously in the Blast this year, reached his first half-century of the season off his 33rd delivery, before McDermott followed him four balls quicker.
Their stand passed Hampshire’s previous best opening stand of 122 – set by Michael Carberry and Mitchell Stokes against Middlesex in 2006 – but two runs later it was ended when Henry Crocombe bowled Vince. It was the young fast bowler’s first of three wickets in an impressive outing.
That started Sharks’ middle-order fightback as Hampshire lost four wickets for 23 runs. Sussex’s brilliant catching was a key reason for the momentum shift; a boundary juggle from Fynn Hudson-Prentice and Obed McCoy’s spectacular flying catch made up for some otherwise below-par ground fielding.
Hampshire accelerated towards the end thanks to Toby Albert’s inventiveness and Liam Dawson’s late swinging to reach their highest Ageas Bowl total since they smashed 202 in 2013.
Sussex’s chase never got going. Hawks were patient before they made their breakthrough in the fourth over when Luke Wright spliced to mid-on before Ali Orr skied to keeper McDermott in the following over.
Hudson-Prentice laboured to eight from 14 balls before he top-edged James Fuller. As a point of comparison, Hampshire were 98 without loss at the 10 over stage, the Sharks were 59 for 3 and statistically given a 1% chance of winning.
Ravi Bopara holed out but Seifert and Rawlins had fun with their 50-stand coming in 24 balls, with the New Zealander plundering a personal half-century in 36 deliveries. Rawlins was run out and Harrison Ward castled either side of a short rain delay. With the game already lost, Seifert plundered three straight sixes in the final over to collect his second T20 century off 55 balls.