Tottenham invested heavily in the close-season to bring the likes of Erik Lamela, Christian Eriksen and Paulinho to White Hart Lane, but they trail fourth-placed Manchester City by seven points with just eight games remaining in their Premier League season.
Furthermore, Manuel Pellegrini's men have three games in hand, and are among the favourites to lift the Premier League crown.
A 1-0 loss in the north London derby on Sunday followed a 4-0 rout at the hands of Chelsea, meaning Tottenham will likely have to settle for another campaign in the UEFA Europa League.
Centre-back Vertonghen conceded Tottenham have failed to improve on last season's standards, but retains belief that the current squad can qualify for Europe's premier competition next term.
"I do not want to look at the table anymore because the gap (to the top four) is very big," he said.
"All four sides at the top are stronger than they were last year and we have stayed at the same level. If you see last season, then we did not improve.
"I think we do have the players to achieve Champions League (qualification) and I hope we can achieve it next year."
