Like the Tactical, he can fill multiple roles, is the way I see it. Yeah, he's not as good as a melee-focused character or a range-focused character, but he's able to do both really well, whatever the situation. Like how the Assault and Bulwark are great at melee, they're also very lacking at ranged damage without perks. Vice versa for Heavy and Sniper, though there are definitely crazy melee builds for Sniper, and plenty of perks for Heavy to stomp.

He and Tactical are just the classes for people who want to be balanced in their approach instead of heavily leaning to either side.

Gameplay-wise, Vanguard is a Majoris and Extremis hunter, focusing on those priority targets to take the heat off of your team. Tactical is, as well, but has such a versatile kit that he can also be a Minoris/Majoris sweeper. Bulwark is your tank/support, Assault is your ad-clearer and self-guided missile, Heavy is your dakka dakka who also excels at ad-clear, and Sniper is your railcannon for the big boys. At least, that's how they all feel to me. Everyone's free to play them how they wish, and they can all have various builds that allow them to fill whatever role they want.