2. Restricted free agency is unpredictable.
Waiting until next summer to lock up Vassell could be risky for several reasons. One is that another team could offer more than what the Spurs intend to offer, or they could try to offer a poison pill contract. In that case, they would try to offer a contract that would hurt the Spurs financially.
That is harder to do since the Spurs don't have a lot of money committed long-term, but another team could technically offer a 3-year, $100 million contract. That would pay him more annually, which could affect future cap space and have him hit unrestricted free agency a year sooner. That is something that San Antonio should want to avoid if they possibly can.
Signing him to a longer contract is better and more advantageous for the Spurs, and the only way to ensure that that happens is to extend him this summer. Although Vassell could have a future with the team, the Spurs ended up flipping Murray and White. Signing him this summer gives San Antonio more options and prevents another team from trying to steal him away in restricted free agency.