Vice's entirely character is just there to be a supporting crutch for Teana. That's it. After this series he'll be lucky if they so much as mention him.
He's just there to give Teana someone almost normal to relate to in a squad full of monsters, offering advice and comfort where he can to help fuck up the yuri fags by adding another possibility. While it's pretty obvious rainbows are straighter then Subaru, Teana (much like Nanoha) has that uncertain element about her added to stop it from being a complete dykefest.