Spurs won 1-0 at Crystal Palace on Sunday thanks to Roberto Soldado's successful spotkick.
"We were very dominant, but we kept them alive and they had a big opportunity in the 89th minute," Vertonghen said.
"We should have more of a killer instinct. If we did we could have scored two or three more, then we would have been able to relax a bit and make more substitutes.
"We were happy with the win. It would have been a better result if we had got two or three goals more."
For the big stories and the banter follow us on Twitter: @tribalfootball