The 32-year-old has completed a return to Spain's top flight following two deeply disappointing seasons in the Bundesliga with Hamburg and was unveiled to supporters and the media on Tuesday.
The hugely-experienced Netherlands international made it clear he still has lofty ambitions after agreeing a deal with the newly-promoted side.
"I spoke with the coach and we want to win the league," he said wryly to reporters.
"The most important thing is that the fans are happy because we're playing good football. When the club dropped to the Segunda [Division], I saw they still had 40,000 fans on YouTube, and that says a lot about the club.
"Betis are a great club and I'm happy to return to the best league in the world."
Van der Vaart, whose mother is from Cadiz, spent two years with Real Madrid before joining Tottenham for a successful two-season spell in the Premier League.
And the Ajax youth academy graduate is now eager to secure a return to the national set-up by impressing for Betis.
"I have huge ambition. In football, one day you're king, another you're bad. I want to return to the Holland side," he said.
"I'm a different footballer to the one who played at Real Madrid. I've got more experience, but I suppose that I'm basically the same type of player."
In an annual survey by German publication Kicker, Van der Vaart emerged top of a poll asking for Bundesliga players to nominate the most disappointing players of the season.
"I had two bad years with Hamburg but I think I'm still a good player. The Kicker piece doesn't bother me," he said simply.