It's a flip effect, so you either need your opponent to attack it while it's face-down or you need to flip summon it after setting it. Ideally, it works like this:
Set a vermin -> opponent's turn -> opponent attacks it -> special summon another one -> your turn -> flip summon it -> special summon the third one -> tribute summon BEWD
2 completely different effects. Thunder Dragon doesn't require you to set the card and be flipped to thin the deck. You can activate thunder dragon's effect from your hand, and play a different card. Bubonic only has a place in maybe stall/burn type decks, or decks that use big tributes.
Honestly, this thing is probably better than a 7/10. Makes tribute summon so easy. I built a deck around gilford the lightning with it, and between this thing, stray lambs, and joey's reinforcements skill, it's not hard for me to draw gilford whenever I need him and make a big comeback.