Raikkonen had said on Thursday he thought Lotus would struggle, but in second practice they were the world champions' closest challengers, 0.663secs slower than Vettel.
The Finn was running a car with a longer wheelbase, aimed at producing better downforce and improved braking stability, while Grosjean was in the standard car - but they set identical lap times to the thousandth of a second.
2012 Lewis Hamilton - McLaren
2011 Sebastian Vettel - Red Bull
2010 Fernando Alonso - Ferrari
2009 Rubens Barrichello - Brawn
2008 - Sebastian Vettel - Toro Rosso
Alonso has admitted he must beat Vettel this weekend and reduce his 46-point deficit in the championship, but on the basis of lap times on Friday that will be a tough ask.
BBC 5 live analyst and former F1 driver Allan McNish said: "Right now I have to say I don't think Ferrari have the absolute pace of the Red Bull.
"I think it will be a lot closer in qualifying than the 0.6secs gap we see between Vettel and the rest on Friday but it really needs to be if people are going to try to challenge him in the race."
Hamilton comes to Monza on the back of four consecutive pole positions, but he seemed to struggle for pace in the second session - nearly 0.9secs off Vettel and 0.027secs ahead of Rosberg, who split him from Button.
Italian GP: Practice one highlights - Hamilton fastest
BBC F1 analyst Gary Anderson said: "It's Friday, so we have to take the times with a small pinch of salt, but whoever is quick, is quick, and it looks like Red Bull are in a fairly dominant position, which is not great for everyone else.
"I still think there is something in the Ferrari, though. And I expect the lap times to be closer in qualifying than they are today.
"Interestingly, the Red Bulls are running with a greater disparity of aerodynamic set-up than I have seen for some time. Vettel appears to have less front wing and more rear wing and Webber the other way around. It will be interesting to see if they converge on Saturday - as in fact they did in Spa last time out."
Ferrari have been fined 10,000 euros for an incident where Felipe Massa nearly hit team-mate Fernando Alonso at the exit of the pit lane after the session had officially ended.
Saturday 7 September : Final practice at 09:55 BST on BBC Two, Radio 5 live sports extra, plus live text commentary online. Qualifying at 12:10 BST on BBC One, Red Button, Radio 5 live plus live text commentary online
Italian Grand Prix, day three
Sunday 8 September: Race coverage at 12:10 BST on BBC One & BBC Red Button at 13:00 BST, plus Radio 5 live at 13:00 BST, and live text commentary online. F1 Forum, 15:15 BBC Red Button. Highlights, 19:00 BBC Three & BBC Red Button
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