Van Persie was Vermaelen's teammate at the Emirates in 2012 when he made the fateful decision to join their fierce rivals.
"It was a surprise he left," Vermaelen told Arsenal's official podcast.
"I knew he was going to go eventually but to hear that he'd all of a sudden left for Manchester United was a big surprise.
"I wanted him to stay at Arsenal of course, because in his last season with us he was the PFA Player of the Year. He had a great season, the best season of his career, so I wanted him to stay and be a legend.
"Of course, he's strong in his character so he couldn't change his mind. I think that's a good thing because that makes him the person and player he is. You couldn't change his mind, he'd made it up that he was going to go."