Somerset 177 for 6 (Trego 70, Anderson 53, Morkel 3-30) beat Surrey 176 for 9 (Jacks 53, J Overton 4-24) by four wickets
Surrey posted 176 for 8, thanks largely to teenager Will Jacks, who smacked 53 off 31 balls, and the experienced Rikki Clarke (32). Jamie Overton continued his resurgence after injury with four for 24, while leg-spinner Max Waller conceded only 15 from four overs.
In reply, Somerset recovered from a shaky start to reach 177 for 6 with 5 balls to spare, Trego hitting 70 off 54 balls and Anderson 53 from 31 as their fifth-wicket partnership yielded 104 runs in 10.3 overs
It was Somerset's sixth successive group victory and, having not won it since 2005, they are starting to look the team to beat in this season's competition.
Trego said: "I thought our bowlers set the game up for us magnificently. Jamie Overton was rightly man-of-the-match for his four wickets, but Max Waller was also outstanding in conceding only 15 from his four overs."
Surrey got off to the worst possible start after losing the toss. Jason Roy played the opening delivery of the game from Waller onto the leg side and Rory Burns was run out at the bowler's end by Jerome Taylor, having set off for a single.
Soon it was 20 for 3 as Nic Maddinson was caught off a skyer, having been dropped the previous ball, and Roy fell to a brilliant one-handed catch by Taylor at mid-wicket in the same Overton over.
When Ben Foakes fell to Roelof van der Merwe for 13 in the seventh over Surrey were 52 for 4. But Jacks and Clarke launched a spirited counter-attack with a stand of 69 in six overs.
The ninth over of the game from van der Merwe cost 26 as Clarke struck one six and Jacks two to give the innings momentum. By the time Overton had Jacks caught by Tom Abell at deep mid-wicket, the 19-year-old had cleared the ropes six times.
Clarke quickly followed, having hit 3 fours and 2 sixes, and, despite handy contributions from Tom Curran and Freddie van den Bergh, Surrey looked short of a par score.
It didn't appear that way when openers Johann Myburgh and Steve Davies perished to tame shots in the second over of Somerset's innings, bowled by Morne Morkel.
By the end of the power-play the hosts had moved to 47 for 3, James Hildreth having fallen for 14 to a fine diving catch at cover by Clarke to give Morkel (three for 30) his third wicket.
Trego hit Gareth Batty for six, but the off-spinner responded by having Tom Abell pouched at deep backward square and at 55 for four Somerset were in a spot of bother.
But Anderson was soon counter-attacking in typical style, hitting two big sixes in the 11th over, bowled by van den Bergh, while the evergreen Trego had settled in to play with similar panache, reaching his fifty off 34 balls.
Together the pair accelerated their partnership to perfection before both falling at the death as a day that had threatened rain ended in bright sunshine, with home supporters in the 7,000 crowd in jubilant mood.