Lochte, who skipped the 400m individual medley to concentrate on the 100 butterfly, has been appearing in his own reality television show and not fully committed to training since last year's London Games but showed he had lost none of his drive by winning two titles on Wednesday.
On Thursday, Lochte posted the sixth fastest time in the world this year at 51.71 but was beaten by Eugene Godsoe, who touched in 51.66 to book a ticket to the July 19-Aug. 4 world championships in Barcelona.
"I put my head down, and everything started fading out," Godsoe said. "I just went for it."
Vollmer claimed her spot on the plane to Spain with the third fastest time this year, coming from behind to overtake Olympic team mate Clare Donahue at the turn to post 57.53.
"It actually wasn't as fast as I wanted it to be," Vollmer said. "Going into prelims, I had no idea where I was going to be.
I've been all over the map at the Grands Prix, but each day of training has really made a difference for me.
"I just thought about staying light and spunky through the first 50. One of my strengths is that second 50."
In other events, Rachel Bootsma (27.68) claimed an American record in the women's 50m backstroke, beating out four-time Olympic gold medalist Missy Franklin, who was second at 27.98.
"It's my first American record, so I'm really happy," Bootsma said. "I just really tried to focus on my start.
"I have a really quick tempo, so I just wanted to get out of my underwaters and just go."
Kevin Steel won the men's 50m breaststroke in 27.26 ahead of Kevin Cordes in 27.38.
David Plummer posted 24.52 in the men's 50m backstroke, the quickest time posted this year, to beat Matt Grevers in second in 24.77.
Maya Dirado and Elizabeth Beisel provided a thrilling contest in the women's 400 individual medley, with Dirado pipping her rival at the wall by 0.16.
In the men's 400m medley, Chase Kalisz prevailed over Tyler Clary while the women's 50m breaststroke went to Jessica Hardy over Breeja Larson.
Youngster Justin Lynch turned heads in the men's 100m butterfly 'B final' when the 16-year-old beat Michael Phelps' 15-16 National Age Group record.
Lynch posted 52.75, beating Phelps mark at the same age of 52.98 from 2001.
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- Eugene Godsoe