Spurs moved to within three points of Roberto Mancini's leaders with Wednesday's 2-0 win over Everton at White Hart Lane.
City have won just three of their last six Premier League games to enable Spurs to close the gap, and Van der Vaart is confident Harry Redknapp's men are ready to take advantage of the slip-ups when they meet at the Etihad Stadium on January 22.
"They are not so consistent any more," said Van der Vaart. "They have lost a few games recently, and maybe that's a little bit in their heads so we can take advantage.
"It's going to be a massive game and we're really looking forward to it. City have many good players. They have many individual players who can score goals and decide a game.
"But we are more like a team, so it's going to be an interesting game."
Van der Vaart insists Tottenham are a lot better now than the side which lost 5-1 to City at White Hart Lane in August and believes they are now serious title contenders
"We look back at our start to the season and think: 'What if?' But at that moment City were too good," he added. "We are much better now and I am really looking forward to our game against them.
"After next weekend against City we will know where we stand. But everybody in the dressing room believes we can win the league.
"I think everybody knows Tottenham have a great team and if we can keep playing like this we will be one of title favourites."