Joakim Soria pitched a ideal eighth inning in relief of Vargas, and Kelvin Herrera worked around a leadoff walk in the ninth to earn his third save and finish a 5-3 homestand for the Royals.
The veteran is also unbeaten in his last eight home starts. "My command has been really good thus far".
For Bumgarner, he escaped jams in the second and third innings, and then the Royals broke through against him in the bottom of the fifth on a two-out infield single by Mike Moustakas that knocked in Paulo Orlando.
Giants LHP Madison Bumgarner makes his first start at the K since Game 7 of the World Series, when he memorably pitched in relief to help San Francisco close out the series. "I'm very grateful, to feel well as quickly as I did", said Posey, who was hit in the head by Diamondbacks RHP Taijuan Walker on April 10. I know that has a lot to do with some things. "It was just one of those days where I had to grind it out".
Matt Cain went seven innings for the Giants while the San Francisco bullpen threw four scoreless innings.
Salvador Perez added an RBI single in the eighth, the third run surrendered by Giants relievers in the past nine games, and one that gave Herrera some breathing room in the ninth. "It's just another series for us", Yost said. Vargas is 2-0 with a 0.66 ERA in his first two starts this season.
GIANTS KEY TRENDS: The Giants have won four of their last five games against American League Central opponents and are 5-1 in their last six games when their opponent allows two runs or less in its previous contest.
Giants DH Buster Posey gave a fan a nice souvenir when he whiffed on a pitch in the seventh inning and his bat flew out of his hands. Bochy missed the series after having a minor heart procedure.
Giants OF Mac Williamson (quad) began a rehab assignment at Class A San Jose on Wednesday, while OF/1B Michael Morse (hamstring) was batting cleanup as the designated hitter.
Since tearing three ankle ligaments and breaking a bone in his left leg in 2011, the catcher has appeared in at least 146 games in each of the last five seasons.
The Kansas City Royals will look to keep their momentum going after winning 4 straight, including a 1-0 win over the Los Angeles Angels to complete the 3-game sweep.