The three-time world champion was 10th fastest in the afternoon session, 1.4 seconds away from fastest man Felipe Massa's Ferrari.
Vettel emphasised that Red Bull was not in huge trouble, but neither was he content with its speed.
"I think it was a tricky day for us," he told reporters in the Shanghai paddock.
"I seemed to struggle a little more this afternoon, with the gap to the guys at the top being a bit bigger than I would like to see.
"But I think we have two or three things that we need to work on and we should be in better shape tomorrow.
"It is not a disaster. It is not as if we are in the dark, we know a little bit about what we need to do.
"I don't have the answer right now, but we'll look into the stuff that we did because once you run, you try to follow your programme, get as many laps and results as you can, so there's not much time to analyse during the session. That is what we do after the session."
Vettel's team-mate Mark Webber was fifth quickest, 0.7s ahead of the sister car, and felt there was little to worry about beyond tyre wear.
"Today was a good day for us, we had a good amount of mileage which is one thing," said Webber.
"There are still some pretty quick cars out there, so nothing unusual from what we have seen earlier in the championship.
"I think we have a little bit of work to do but the car doesn't feel too bad, we just have to work on the tyres."
- Motor Racing
- Felipe Massa