Vertonghen said: "You go a bit blind and you hope for some contact, and I had it".
Spurs regained the lead eight minutes after Joselu's header.
Newcastle brought on Rondon with half an hour remaining, and he almost grabbed a point in the closing stages, seeing his effort deflect off Vertonghen and onto the bar.
The Argentine added: "We have a very good squad, we have very good players and it is not easy to add players that improve that".
"We deserved at least a point", insisted manager Rafa Benitez, whose side struck the frame of the goal twice in the second half through Mohamed Diame and substitute Salomon Rondon.
"Hugo [Lloris] can be a changed man because Hugo won it", he said.
Kenedy was guilty of squandering arguably Newcastle's best chance of drawing level midway through the second half.
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He'll crack the top 15 in the world rankings next week. "I need to work much more and hopefully I can reach his level one day". Stefanos will play the victor of the semi-final between top-ranked Rafael Nadal and Karen Khachanov .
The Colombian's header down was met at the far post by Vertonghen under pressure from Paul Dummett and the ball ricocheted down from the crossbar, with referee Martin Atkinson awarding the goal after being alerted that the ball had crossed the line.
Yet Newcastle responded right away as Joselu finished Matt Ritchie's cross superbly to send St James' Park wild.
Their glee did not last long, with Alli arriving at the back post to head in Serge Aurier's deep cross for what proved to be the victor.
Mauricio Pochettino admitted he was proud of his Tottenham stars as they put on a united front to start their Premier League season with a 2-1 win at Newcastle.
But Spurs held on for a much-needed three points after recent criticism for not making a signing this summer, and the result means Newcastle, whose fans protested against owner Mike Ashley in the city centre before the game, are without an opening-day victory since 2012.
After a frantic opening at St James' Park, there was more drama to come as in the 18th minute a third goal of the contest was scored.
"The team worked hard against a very good side", the Spaniard continued.
"Davinson had a great touch with the first header - and there could have been another one later because from the other side, he had a first touch again, but I didn't make that run again".