According to a source, Nick and his feisty Canadian fiancee are fighting often, Us Weekly reported.
"It's not a solid relationship, and it won't last," the source insisted.
Nick, 36, and Vanessa, 29, got engaged on The Bachelor's 21st season earlier this year despite apparent jealousy issues and arguing over which country they should permanently live in.
But Vanessa eventually decided to leave her job and her family and move to Los Angeles, where she supported Nick as he competed on Dancing with the Stars and then launched his new line of grooming products, The Polished Gent.
Another source told Us that the pair doesn't get along because they're too similar in that they are both opinionated and have big personalities.
"Vanessa is no pushover. She's not going to just do whatever he wants," the second source explained. "He's attracted to that, but it makes their relationship volatile."
Nick and Vanessa just went to Puerto Vallarta, Mexico to attend Carly Waddell and Evan Bass' wedding, but a witness told the magazine that on their flight home to Los Angeles on June 18, "they looked miserable."
"They didn't exchange five words the entire flight. They were angled completely away from each other," the witness told Us.
So it sounds like Nick and Vanessa had gotten into an argument before they even boarded the plane, and things certainly weren't resolved during the flight.
Upon landing, "Nick grabbed his bag and ran ahead of Vanessa," the witness added. "They really don't seem to like each other."
Vanessa was also reportedly spotted on a date night with Nick in Los Angeles recently not wearing her 3.5-carat Neil Lane diamond engagement ring.