A fortnight on from his maiden victory with Aprilia in the Americas GP, Vinales could muster only a ninth-place finish at Jerez, over 11s off race winner Francesco Bagnaia on the factory Ducati.
He and Aprilia team-mate Aleix Espargaro were also the slowest of the Q2 runners in qualifying, ending up more than two seconds slower than the pace set by Gresini Ducati rider Marc Marquez in 11th and 12th positions.
But having got another opportunity to run the RS-GP on Monday in the official in-season test, Vinales discovered that there was an issue with his second bike that held him back during the grand prix.
“The lap times seemed really competitive, also lap times on rhythm. So, we missed something on the weekend, we need to understand.
“But today the bikes are really good, so we need to understand why it doesn't work that good, especially on Sundays, because the rest of the weekend we very were competitive.
“We need to understand between bike A and B what is happening.
“I did a [1m]36 high on the [long-run] rhythm. In the race, I was doing 37-high in the mediums.
"There is a difference between A and B bikes. Unfortunately, I raced with the bike I felt I'm not competitive [with]. This is something to alert us to really check even the last detail.”
Aprilia had originally suspected a front-brake problem with Vinales’ second bike in Austin, which was fixed before he switched to that version at Jerez on Sunday following his crash in the sprint race.
But it was only when he did a direct comparison between the two RS-GPs on Monday that a more serious problem was detected, which explains his lack of pace in Spain.
Vinales is now confident Aprilia can make a “huge step” forward in coming races, as he prepares for the French Grand Prix at Le Mans on 12 May.
“Because I crashed with bike A on Saturday, so to be safe we went with bike B,” he explained. “But to be honest we didn't realise that it wasn’t working.
“I said at the weekend and also in Austin that I was not stopping with bike B. We related it to a front brake problem and at the end it's not a front brake problem.
“Today we discovered more or less what it is and now they are working similarly. So that's a huge step for the future because in the race I could be much faster.
“So this is something we have to be alert of and every race we will learn more and more, so the team is more prepared.”
Vinales suggested that the problem in question may have been caused by a part missing Aprilia's quality control: "It's a wear issue, I don't know exactly, but it's very possible."
