How we cite our quotes: Act.Scene
Quote #1
Vit. …They told me my intent was to root up
That well-grown yew, and plant i' the stead of it
A wither'd blackthorn; and for that they vow'd
To bury me alive. (1.2)
Maybe the yew Vittoria is uprooting (and which her husband and Isabella want to stay put) is the old order of things—her marriage to Camillo, the Duke's marriage to Isabella, and the right order of their respective family trees? She will betray her husband in order to keep rooting up this symbolic tree.
Quote #2
Flam. He will shoot pills into a man's guts shall make them have more ventages than a cornet or a lamprey; he will poison a kiss; and was once minded for his masterpiece, because Ireland breeds no poison, to have prepared a deadly vapour in a Spaniard's fart, that should have poisoned all Dublin. (2.1)
Flamineo is describing the poison expert, Doctor Julio, who will help Brachiano murder his wife. It's some light comic relief—what with a poisonous fart that could kill everyone in Dublin. That must've been an early incident of chemical warfare…
Quote #3
Brach. Small mischiefs are by greater made secure. (2.1)
Brachiano is explaining that the smaller mischief—committing adultery with Vittoria and leaving his wife—will be concealed by the greater betrayal of actually murdering Isabella. This turns out to be a pretty un-wise, since the murder of Isabella is what makes Francisco so determined to kill Brachiano.