Catcher John Jaso supplied some needed punch with a home run and three RBIs, and the Mariners put together 14 hits and equaled their season-high run total. The offensive burst made a winner of Vargas, who held the hot-hitting Rangers to three runs on five hits over 6 2/3 innings before turning a 4-3 lead over to the bullpen.
Vargas has the third-lowest run support in baseball since 2009, behind only Doug Fister and Johan Santana, but got double-digit help on the 90-degree night in Arlington.
"I felt like our at-bats were as good tonight throughout the game as they've been all year long," manager Eric Wedge said. "Not just with that eighth inning, but all the at-bats prior to that. I felt really good about our approach up and down the lineup. That's what we're talking about. I was really pleased with our offense tonight."
Kyle Seager broke out of a 2-for-28 slump by going 3-for-5 with two doubles to raise his average to .265. Michael Saunders equaled his career high with three hits as well.
And Seattle busted things open with six runs on six hits in the eighth off tough Rangers relievers Alexi Ogando and Mike Adams, including bases-loaded singles by Dustin Ackley and Mike Carp. Ackley is now 8-for-10 with 14 RBIs in bases-loaded situations in his career.
The normally sure-handed Rangers defense also helped out with critical errors on third baseman Adrian Beltre and shortstop Elvis Andrus leading to four unearned runs.
Andrus' bases-loaded error helped in the breakaway eighth, so did a run on a wild pitch. The six runs equaled Seattle's most in an inning this season, as well as the most allowed by Texas.
"We just got outplayed tonight," said Texas designated hitter Michael Young. "That big inning was the separator. It was close up to that point, and then they had some good at-bats against some good pitchers. It was their ballgame after that."
The win was just the third in 15 games for Seattle at Rangers Ballpark since the start of the 2011 season and lifted the Mariners to 22-30 on the season.
Vargas improved to 6-4 with a 3.45 ERA. The lefty took a 4-1 advantage into the sixth, but Josh Hamilton made things interesting with a two-run blast to center field -- his 21st of the season -- to cut the margin to one.
Hamilton already had one RBI in the game, giving him 56 for the season ... or 30 more than Seager's 26 as the Mariners' leader.
Vargas pointed to a bases-loaded double play on a hard shot by Ian Kinsler to shortstop Brendan Ryan in the fifth with changing the game. That play, moments after Ryan lost a pop fly that fell in for a single, kept the Rangers from erupting with their own big inning.
"That was the biggest moment of the game right there," Vargas said. "For [Ryan] to keep that in front of him, let alone turn the double play, that turned the page in the game for us."
Ryan smothered the hard-hit ball after it bounded off his chest, pushed it to Ackley and then watched as his second baseman avoided a hard slide by Craig Gentry to complete the throw to first.
"That might have been our best double play we've had because that was a tough feed, he got rid of it with a guy coming in hard and it was an outstanding spin by Ack," Ryan said. "I needed a two-hand shovel pass, just trying to get it there. I knew if I got it to him, we had him at second. But I didn't know about first. That was going to take a great spin and that was every bit of that."
Meanwhile, Jaso supplied a little offensive pop with a two-run home run in the second off Rangers starter Scott Feldman, a 405-foot shot just over the fence in center field for a 2-1 lead. It was Jaso's second long ball of the season.
The Mariners then took advantage of a rare misplay by Beltre to score twice more in the fifth, with Beltre throwing wide of first baseman Mike Napoli after snaring a ground ball to his left by Jesus Montero with two outs and the bases loaded.
Ryan and Ackley, who'd pulled off a double steal prior to an intentional walk to Ichiro Suzuki, scored on the play to give Seattle a 4-1 lead. Then the floodgates opened in the eighth as the Mariners put together six hits and Andrus' error with their first seven hitters.
"That's something we need to do as an offense," said Seager, who led off that rally with a double. "Every once in a while you break out and it lets the staff kind of breathe a little and know they don't have to be perfect."