The Belgium international centre-back completed his switch to the Liga giants from Arsenal on Sunday, with Barcelona confirming a €10 million fee was paid with up to €5 million due depending on his own personal performances, while a possible €3.8 million could be due from Barca's achievements.
Vermaelen had been heavily linked with a switch to Old Trafford throughout the close-season, with new United manager Louis van Gaal reportedly keen to bolster his defensive options.
However, the 28-year-old revealed there was only one option once he heard of Barca's interest.
"Coming to Barca is a dream come true," he said at a press conference. "It wasn't a difficult decision at all.
"I'm very happy to be here, for a lot of players it's a dream to play for this big club. It's always a big challenge but I look forward to it.
"I will be playing with the best players in the world, training with them every day and I hope to get better as a player.
"You know when you come to a club like this they have world-class defenders. There's competition, I'm excited, it's down to me to show I can play for this club."
"Last year was a bit disappointing with injuries," he added. "It happens sometimes in a career with a player. I hope to come here and hope that I can be strong here and play all the games.
"I'm confident with the medical team here I can be fit and play a lot of games."
