Berkshire left armer Smith ended with four for 10 to back up a match-winning 99-run stand between New Zealander Suzie Bates and Arran Brindle in front of 1,000 fans at the Ageas Bowl.
Vipers set 119 to win, before bowling the Diamonds out with four overs to spare – the Diamonds the first club to be eliminated from the Super League.
Charlotte Edwards was not able to bat in the opening game after colliding with teammate Fi Morris, which subsequently forced her to miss the win against the Thunder.
She finally got out to the middle after winning the toss and batting, but her innings was short lived as she edged former England teammate Katherine Brunt’s away swinger behind.
Next over and the Vipers were two down when pacy Shabnim Ismail struck with her first delivery, Izzy Collis lbw.
The Diamonds had the hosts on the ropes on 9 for three, when Sara McGlashan pushed to Hollie Armitage at first slip, and then 19 for four as Lydia Greenway, who had been dropped at first slip, reverse swept to Ismail at short third man.
But international veterans Arran Brindle and Suzie Bates turned the innings around with a much needed 99 partnership, on a tricky to bat on slow track.
The ICC T20 No.2 ranked player Bates slowly batted herself in and was rewarded by being able to play her shots at the end.
She was finished up with an unbeaten run-a-ball 45, having smashed the final delivery for a six over mid-wicket, Brindle an equally handy 45 to take the Vipers to a competitive total of 118.
Lauren Winfield got the chase off to a good start by rocking onto the back foot and hammering a pair of boundaries off Morna Nielsen.
But the England opener drilled straight to Brindle at mid-wicket without scoring another run, and Edwards ran out Armitage to leave the Diamonds struggling on 19 for two.
Wickets continued to fall as Smith deceived Alice Davidson-Richards with the flight to have her stumped.
Beth Mooney, who kept the score ticking with 17 was dropped at mid-wicket by McGlashan, who required a visit by the physio, but Alex Blackwell was brilliantly caught and bowled by Smith.
Mooney was bowled backing away to Nielsen before Jenny Gunn was bowled by Smith.
Medium pacer Smith, on her second appearance, ended with figures of four for 10 when she had Brunt caught at backward point.
Stephanie Butler was caught behind and Katie Levick spooned to Brindle to be bowled out for 64 and hand the Vipers another home bonus point win.
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