Vespids are poor creatures. They were obviously designed with 4th ed in mind (along with the rest of the tau codex)
They have a relatively good gun, which they can't use, as their unit sizes are too small, and their
BS is too bad. They are obviously supposed to beat up marines, but their guns aren't good enough, they aren't good enough in
CC, and they have a save that a boltgun takes away. Very bad design I'd say.
How to fix them? I'd say that they at least need carapace armour, and they need better assault capability. How I'd do it, meh, I don't know, I'm not a designer...