I posted this elsewhere as well, but I think that Lauren was sent by someone (Irina?) to keep Sydney away from Vaughn during her missing year, not to gather any CIA intel.
Whatever Syd was up to and for whatever reasons, THEY (whoever "They" are) needed to make sure she could not contact the two most important people in her life: Jack and Vaughn, which of course she would try to do. So they sent Jack to prison and Lauren to Vaughn. Remember when Kendall kind of "sent" Syd to see Vaughn and she just so happened to see the two of them together? I smell a set-up. And even if Sydney had tried a little harder to talk to Vaughn, Lauren would have been there to intercept her.
And I have my own theories about who "They" are...and about Kendall!