According to Adam Schefter of ESPN, the Saints linebacker is having knee surgery today.
Saints LB Jonathan Vilma undergoing knee surgery today in Philadelphia; out for preseason, should be back for opener.
— Adam Schefter (@AdamSchefter) August 15, 2013 
Vilma is expected to miss the rest of the preseason, but is expected to be ready for the regular season opener.
Source: Darin Gantt, NBC Sports Pro Football Talk