Lewis Hamilton and Jenson Button say Red Bull are "very fast"
McLaren fear Red Bull will be out of reach at the Chinese Grand Prix.
Lewis Hamilton was 0.166 seconds behind Vettel's leading time with team-mate Jenson Button 0.247secs off the pace as McLaren ran an updated car in Shanghai.
"My feeling is that the Red Bulls are very, very quick this weekend," said Hamilton, who qualified behind Vettel at the last race in Malaysia.
Button added: "I'll be surprised if we can fight for pole position in qualifying as they look very fast."
In an effort to close the gap on Red Bull, who won the opening races in Australia and Malaysia, McLaren tried out an updated exhaust system, floor and a new front wing in China.
But the drivers were uncertain after Friday practice whether the upgrade had added any value and said the team would have to analyse data before deciding whether to run with it during qualifying at 0700 BST on Saturday.
"My gut feeling is that's it not [an improvement]," said Hamilton. "I don't think it's as good as [what] we had before.
"This morning's session was difficult. I had all the bits that they were changing with the rear and floor.
"I had a big off moment and that was partly to do with one of those things not working.
"In the afternoon we chose to go with the upgrade we had but I've been really struggling with the balance so I don't know if it's the upgrade.
"I was able to get a reasonable lap time but I wasn't comfortable in the car."
Button said he did not know if the team would ultimately decide to stick with the upgrades and when asked if he thought McLaren had gained ground on Red Bull he answered: "No, not at all.
"If we sort our balance out we'll be closer. I'll be surprised if we can fight for pole position in qualifying as they look very fast.
"The car is still a good car to drive but we didn't improve the balance and we will look at why the changes we made didn't give us the improvement. "
Although McLaren were not able to match Red Bull over a single lap at the last two races, they were generally believed to at least be a match for Red Bull on pace in the race.
However, on Friday's evidence, Hamilton believes that is not the case in China.
"Through the long run it looked like they were quicker than Jenson, maybe a second compared to his long run on soft tyres," said Hamilton, who added that he had done his long stints on hard tyres.
"There is quite a bit of downforce on this circuit and they've shown time and time again that they generally have a little bit more than us.
"We are just behind them but there is a little bit of gap between us. We'll still be chasing them and we'll keep pushing."
Hamilton is contracted to McLaren until the end of 2012 and was quoted in British newspapers on Friday suggesting that he might leave the team if they could not provide him with a car with which he can win titles.
On Friday, however, Hamilton told BBC Sport: "There is no question of my loyalty for my team. It's been my team since I was six years old.
"I'm here, I'm committed to them. I want to try and help them as well as becoming one of the most successful drivers myself. I want them to be out there to do that for me.
"If you look at our performance over the years I think you can say I'm the only who has won a grand prix every year, so I've had a winning car every season. No other team can say that for the last five years. That's quite a positive thing."
McLaren technical director Paddy Lowe said the team would spend the night analysing the data before deciding whether to abandon the updates.
"We've got a lot of numbers to look at it and we'll be deciding what to fit," Lowe said.
"I'm sure some of these things we've tried will have worked. There is no black or white answer because we have quite a few different pieces to try.
"I certainly don't think Red Bull are cruising. They are feeling the pressure."