A source close to the couple told Australian tabloid, NW, ""One of the reasons Nick and Vanessa's romance works is that he is the bigger star. Jessica's fame ruined their marriage and now Nick can't help but worry the same thing will happen to Vanessa."

Vanessa has even reportedly turned down a marriage proposal from Nick because she didn't want her love life to get in the way of her career. A friend of the couple says, "Vanessa has dreamt of being a movie star since she was a little girl. Now that she's dating Nick, though, she's actually getting offers - she's in no rush to become Mrs. Nick Lachey."

Vanessa's dream is coming true as she has landed her first big film role and will star in the Fantastic Four sequel alongside Jessica Alba and Chris Evans. Complicating matters between Nick and Vanessa is the fact that she has feelings for Chris. Another source reveals, "Vanessa's had a crush on Chris in the past and Nick knows about it. Back then Chris was dating Jessica Biel so nothing became of it, but now he's available and that scares Nick."

If Minnillo were to date Evans, it wouldn't be the first time Lachey lost his woman to her co-star. It's widely believed that Nick's ex-wife, Jessica Simpson cheated on him with her Dukes of Hazzard co-star, Johnny Knoxville. As Nick is said to have told a friend about Vanessa, "It's the Jessica syndrome all over again."