Although Vettel finished on the podium in the Malaysian GP, he was only sixth and fifth in the following races in Bahrain and China, and had to move aside for team-mate Daniel Ricciardo on both occasions.
When Red Bull confirmed that Vettel would get a new chassis for Barcelona, the team indicated that the change had long been scheduled rather than being a reaction to his form in the opening races.
OPINION: Why F1 should still fear Vettel
But Vettel said the decision was made after Shanghai and as part of efforts to ensure every possible cause of his troubles was investigated.
"I think we concluded after China where we were quite a little bit behind to change the chassis," he said.
"It is not a new chassis, it is an old one we used in testing in the winter and we have some experience with it.
"It is just to try everything we can.
"It is not unusual to change chassis generally, we decided to change back to an older chassis just to make sure that nothing is wrong.
"It is just to see the changes, reset everything and try again. We don't think there was anything wrong with the old chassis but nevertheless we decided to change. We should get an answer this weekend."
Vettel said that rear-end instability was part of his current issue with the RB10, but that there was no simple answer to why he had been off his usual pace so far in 2014.
"Generally we all have our own style of how we like to drive the car and how you learn to set up the car," he said.
"In general I don't mind when the rear is moving, I don't mind oversteer, but if it is too much or it starts to bother you, and car slides too much and you find yourself correcting rather than pushing and getting maximum out, it slows you down.
"That is part of the problems so far. It would be nice to have one problem and one fix behind it, but it is more complex this year, there are more factors than car set-up."
This week's AUTOSPORT magazine includes an in-depth analysis of the Red Bull intra-team battle and why Vettel has come under threat from Ricciardo
- Daniel Ricciardo