Fortunately, D&Ds vermin tend to be somewhat more varied than its animals. Unfortunately, and in blatant disregard of the square-cube law, they also tend to come in half a dozen sizes. This'll be fun.

Without further ado, let's review the Monster Manual's final critters!

Giant Ant, Worker

2 vermin RHD (somehow even worse than animal RHD), awful stats (no physical bonuses whatsoever and no intelligence?), fast land speed, slower climb speed, some neat natural armor, a moderate bite attack with Improved Grab, Scent, and Track as a bonus feat. I'm not sure I'd give this +0 LA if it'd been intelligent, let alone as it is now. -0 LA for sure.

Giant Ant, Soldier

Still 2 RHD, but better than the worker in every way. There's +4 strength, +2 constitution, +2 wisdom and +2 charisma, a somewhat stronger bite, and the ability to sting grappled foes for a low amount of acid and piercing damage. I'm fairly sure this is still -0 LA.

Giant Ant, Queen

Don't assume these are lumbering noncombatants: giant ant queens are slower than their children, but still significantly faster than a human, and their strength, size and bite attacks are unrivalled by any of the ants reviewed before. That said, they come with 4 RHD, and frankly don't have much going for them other than the bite. -0 LA once more.

Giant Bee

While their 80 ft. fly speed may seem attractive, giant bees suffer from a few minor flaws, such as their mindlessness, lack of manipulators, and inability to use their natural attack more than once before dying. They have the questionable honor of being the only MM monster find that can actually be defeated by not doing anything (as opposed to those were inactivity only results in stalemates, like toads and shriekers). The -0 LA is self-explanatory.

Giant Bombardier Beetle

Another 2 RHD creature. Ability scores are average, natural armor is decent, bite attack is on the weaker side, and there's no special movement modes. At first glance, these guys are inferior even to giant worker ants.

They're somewhat redeemed by the unique and interesting ability to spray acid once per round, but to be honest it's so weak and affects such a small area that unless you're facing half a dozen swarmfighting goblins you'd probably be better off being a normal melee guy with an actual weapon. -0 LA once more.

Giant Fire Beetle

1 RHD: we're off to a good start. That said, their ability scores are terrible, they have no racial features other than a moderate bite attack and an illumination ability equivalent to a 1 GP item. Don't play this until you want to be a glorified torch with teeth, -0 LA once more.

Giant Stag Beetle

Large with 23 strength, but 7 RHD. Compare them to bears, which are stronger, have more attacks, and have less HD, and the -0 LA should be obvious.

Giant Praying Mantis

4 RHD, flight, a considerable strength bonus, Large size, and two natural attacks. It's still -0 LA, but at least it's getting closer to a viable character.

Giant Wasp

5 RHD and melee abilities inferior to the praying mantis's. The poison is neat, but not enough to salvage this from -0 LA.

Monstrous Centipede, Tiny

Tiny, with less than 1 RHD, a dexterity bonus, huge strength penalty, and no intelligence at all. Climb and walk speeds of 20 ft., and an incredibly weak bite attack with what's probably the weakest poison of any monster ever. Yeah, no: -0 LA.

Monstrous Centipede, Small

Tiny centipedes, with more speed and strength as well as slightly stronger poisonous bites. I mean, it's still a negligible amount of dex damage, but it's getting better!

Still -0 LA though.

Monstrous Centipede, Medium

Same increases as before: four extra points of strength, 10 feet of movement, and an increase in bite damage and poison dice (it now deals a hefty 1d3 dexterity damage). I want to rate those +0, if only to have vermin with different level adjustments, but I honestly can't justify it to myself.

-0 again.

Monstrous Centipede, Large

3 RHD, with an impressive 13 strength and 1d8 bite. -0 LA.

Monstrous Centipede, Huge

Typing this all out is taking more time than coming up with the LA. 6 RHD? 17 strength? Ew: -0 LA.

Monstrous Centipede, Gargantuan&Colossal

Because why not go the dragon route. Combining useless monsters ftw!

Anyway: both of these, the Colossal variant especially, are unusable as PCs. The CR/HD imbalance is simply too great: both deserve -0 LA.

Monstrous Scorpion, Tiny

Why the hell did vipers get poison that's equally damaging across all size categories, but not the monstrous vermin? How are they different?

Monstrous scorpions have two claw attacks, as well as a sting that delivers a weak constitution-damaging poison. Anything hit by their claws can get grappled and constricted, with the scorpion using its dexterity rather than its strength modifier. They also have Weapon Finesse as a bonus feat, and get tremorsense. Could this be turned into a serviceable rogue build? I think so. Perhaps it won't be the strongest, but it could be made to work. +0 LA, which is probably at least partially my aforementioned desire to have variable LAs.

Monstrous Scorpion, Small

Basically slightly bigger tinies with stronger poison and more strength. Sure, that's worth a +0.

Monstrous Scorpoion, Medium+

No, for reasons that should be obvious by now. -0 LA.

Monstrous Spider, Tiny

Weak strength-damaging poison, a climb speed, free Weapon Finesse, and a web-creating ability that's honestly pretty interesting (being usable as anything from traps to movement to enemy detection to improvised nets). I think +0 LA could be justified here, though that's in no small part based on my one-time tiny monstrous spider character.

Monstrous Spider, Small

Tiny spiders but better in almost any way. Sure, that's worth +0 LA.

Monstrous Spider, Medium+

And we're back to the unplayable things. -0 LA.