Some cards are just made with a metallic look.
That's all there is to it. I have many cards with a metallic sheen on the space around the Pokémon (attacks, HP, etc.). As of Black & White, I believe, they may also have silhouettes of their Energy type is the background.
Some cards are from certain events/championships.
Special cards, such as the ones from McDonalds, ones that are special promos, and one-of-a-kind reward cards from Tournaments and Championships have a metallic sheen on the Pokémon itself. This may be the case with your Vibrava.