I think television shows really have a hard time keeping track of plot points, with over 20 episodes lasting nearly a full day's time over a season. That could be the case here, though, I think Lauren and Vaughn actually did legitimately fall in love and get married for that reason. You can tell that Lauren has conflict about her deeds when Vaughn is involved, and was actually really upset about the Vaughn/Sidney connection. I'm not so sure that the marriage was set up so that she could gather information, simply because she already was inside the CIA's inner circle and probably wouldn't need Vaughn's help. Vaughn's decision to go back into the CIA is what caused all the conflict in the first place, which set up a really neat parellel between Jack and him (marrying the double agent, the lies, the emerging of a monster inside after the discovery is made--see the last episode where he starts pouring acid on Dr. Jong Lee (perennial Asian bad guy)).

If I'm wrong, I think it's simply a misstep, where they forget about all the logic--look at "24" and how they constantly introduce and drop subplots. This season had President Palmer with a girlfriend (Wendy Crewson) in the first couple of episodes, and it was causing strain on the re-election--it's vanished (So had a dumb subplot involving a mysterious baby being brought and dropped off at CTU that belonged to Jack's partner Chase). Kind of like the 1st season, which began after 9/11, Mia Kirshner blowing up a passenger plane and parachuting out. There's no mention of the plane's demise in later episodes. I think that a television show has the luxury of figuring out what's not working and sweep it under the rug (they of course don't film all the episodes for a season and cross their fingers, they film several and then take a break and film some more later (or shoot as much as they can and cut out the "bad" stuff), by then they can sort of get the idea what viewers like and dislike), because if they can cut it to the bone and make it about things people care about, then no one will worry about the other stuff.

Those are my ideas about the matter. Obviously, if there was some darker reason Lauren married Vaughn, then they made a slight error. I'm just upset that I have to wait three weeks for the finale, then wait a whole summer for the explanation of what is sure to be a bombshell.