Both

He used that dream to have a hope for a brighter future than just being a "street rat" in Rabanastre. He wanted to go the opposite way from Reks who joined the law (army) since Reks ended up badly.

After many adventures Vaan didn't want to get back to being a "street rat" and still wanted to travel and have adventures and he also got to be the caretaker for the Strahl. So being a pirate was both easy and also allowed him to do what he wanted.

What started as a dream to escape the burdens of a life without hope turned into a career....

Just my 2 cents...