Vermaelen, who spent most of last season on the bench as Per Mertesacker and Laurent Koscielny formed a centre-half partnership, has been linked with a move to Manchester United.
And speaking after Arsenal's pre-season opener at Boreham Wood, where his side won 2-0, Wenger said: "I cannot guarantee he will stay, but should he go I would bring another centre-back in for sure."
Wenger has already recruited Alexis Sanchez and Mathieu Debuchy this summer and will "definitely" bring in a goalkeeper to compete with Wojciech Szczesny.
Real Madrid midfielder Sami Khedira is another reported target, and Wenger said: "It's not over, there's still a long way to go.
"I can't give you any names because you're never sure if you can do what you want, but we're still out there to strengthen our squad."
Aaron Ramsey was among the senior players to feature in Saturday's friendly - decided by Kris Olsson's goal and a Benik Afobe penalty - and there was an appearance for Abou Diaby, who missed all but the tail-end of last season with a knee injury.
Wenger said: "We have big competition in midfield, but we have many players who didn't play many games last year - Alex Oxlade-Chamberlain, Jack Wilshere, Abou Diaby.
"They're back now but it's very difficult to predict how fit they will be at the start of the season."