If it is the plastic attached to the tailgate, their are a series of panel pins that can be replaced. The Vibe used the same pins throughout the interior doors and side trim panels. If it is the plastic cover that fits inside the panel, you can heat the cover and bend the tabs to better engage the tailgate plastic panel.
2005 #10,524 Neptune Vibe "Bandit" Auto, Moon & Tunes 207k
1991 Pontiac Bonneville 3.8L "Granny's Whip" 136k