Ricciardo and Vergne are vying with Kimi Raikkonen for the Red Bull seat alongside Sebastian Vettel that Mark Webber will vacate after this year.
After Webber's announcement at Silverstone, Ricciardo proceeded to qualify in the top six for the next two races, and finished eighth in Britain.
Vergne could not get beyond Q2 at either event and retired from both grands prix.
But their form had been the other way around in Monaco and Canada, where it was Vergne who flew into the points as Ricciardo languished in the midfield.
"Before all the Red Bull talk, I had come off two poor races," said Ricciardo.
"We had three weeks off after Montreal and that was a good time to really reassess a few things and try and bring the boys together and myself and really try and understand how we can move forward.
"That alone, and the fact that JEV had two really strong races, which showed the car had potential, gave me a lot of fire."
Ricciardo said the difference had not come from how hard he was pushing on track, but from working better on his preparations.
"I like to think I always work hard away from the cockpit, but maybe I worked even harder," he said.
"That was the real motivation to make me step up, the Red Bull thing was just a bit of icing!"