Tossing 4 1/3 innings against the D-backs on Wednesday in the loss, the Mariners left-hander surrendered a career-high 10 runs on nine hits, five of which were home runs.
A noticeable trend for Vargas this season, he's given up long balls in eight straight starts and a total of 20 on the season.
"The biggest thing with him is being aggressive with his fastball, being aggressive with his secondary stuff, in particular his changeup, and just throwing it the way he wants to throw it. He's such a command guy, but he has to be aggressive with that, as well," Mariners manager Eric Wedge said. "I think he got a little bit away from that his last start."
He'll try to limit the home runs and stay down in the strike zone come Tuesday against the A's at Safeco Field.
Meanwhile, left-hander Travis Blackley will be on the hill for Oakland in his first career start against Seattle.
Coming off the best start of his career, Blackley limited the Dodgers to just one run on three hits in eight innings of work Thursday. In complete command of his pitches, Blackley struck out six and walked none, earning no decision.
Blackley faced two over the minimum in his eight innings, less than 24 hours after fellow southpaw Tommy Milone stifled the same Dodgers lineup with a complete-game effort.
"We didn't have guys in scoring position," Dodgers manager Don Mattingly said after the game. "You can't really expect to win if you don't get guys on base. We got chances and didn't execute."
A's: Cespedes to return to outfield Tuesday
Yoenis Cespedes will be penciled in to start Tuesday's game against the Mariners in the outfield after six straight games at designated hitter.
Nursing a sore left hamstring the last few weeks, Cespedes will either be in left or center field, depending on whether Coco Crisp gets a break from the latter spot.
"Sometimes he looked like he'd rather be out on the field and wanted to do more," manager Bob Melvin said. "For a guy who's used to being on the field, the time between at-bats when you're DH'ing can be a challenge, but I think he's been reasonably good at it."
Mariners: Seager returns from calf soreness
Third baseman Kyle Seager was back in the Mariners lineup Monday after being limited to a pinch-hitting performance Sunday due to a sore calf.
Seager fouled a ball off his leg in the seventh inning of Saturday's game at San Diego. He finished that game, but wasn't at full speed on Sunday.
"That wasn't the best one. That one hurt a little bit," Seager said Monday. "I squared it up pretty good. But it's feeling better today."
A's lefty Brett Anderson is with the team on its current seven-game road swing through Seattle and Texas and will throw his second bullpen session on Wednesday.
Charlie Furbush has recorded at least one strikeout in each of his last 16 games, the seventh-longest streak by a reliever in Mariners history.