The gimmick behind the wimpiest Pokémon is that at a certain point (usually Level 20ish) they evolve into something that is the complete opposite of what they were.
-Magikarp into Gyarados is the most famous since it's the first. High stats, a solid move pool, but its typing of Water/Flying makes it a bit of a glass cannon. Though they did fix that in Gen 6 with Mega Gyarados and its Water/Dragon typing.
-Feebas is a pain in the arse to find, because it only appears when you fish in 6 specific tiles of water in one route, and those tiles randomize every day. And to make it evolve, you have to max out its "Beauty" stat used for contests. (Later games added a new hold item, the Beauty Scale, that evolves it while being traded and holding it). But when it evolves into Milotic, it is a total tank. It will shrug off even super-effective electric attacks!
-Wishiwashi doesn't evolve, but its ability activates when it is Level 20 or higher; it summons a bunch of Wishiwashi to form a school in the shape of a giant murderfish and boosts its attack and defense stats (normal AND special) up to levels past various legendaries... but its speed and HP are still meh.
-Wimpod is literally the wimpiest Pokémon, it is the Turn Tail Pokémon and its ability is called "Wimp Out". It has garbage stats (except for speed), and it only knows Struggle Bug and Sand Attack. But at level 30, it becomes Golisopod, the partner Pokémon of Team Skull's Guzma. It is proficient in physical attacks, and gets a crazy good movepool of strong physical attacks. It even has a signature move, First Impression, a 90-power Bug-type move that has +2 priority (but only works on the first turn it is out). Though it still has the Wimp Out ability, only that it has been renamed and re-explained as something less wimpy... "Emergency Exit", yeah.

If Vincent leveled up and evolved this team, he'd be a dangerous Water-type user.