What's the purpose of these blue double V collectibles?

Every skate park in the game has secret collectibles. Collect a set from each park and you can clear some challenges.

The "Vicarious Visions" logos are just another one of the more obvious sets. Collect them all at each park and you get a profile logo, some experience, and cash.

Full listing of challenges could be found in the "Skate Park Challenges" under the "Secrets" tab.

Be warned that all secret challenges does not show you the progress for each park. So you'll need to keep track for yourself which parks you completed the challenge for.