Symbolism, Imagery, Allegory
Vit. A foolish idle dream:
Methought I walked about the mid of night
Into a churchyard, where a goodly yew-tree
Spread her large root in ground: under that yew,
As I sat sadly leaning on a grave,
Chequer'd with cross-sticks, there came stealing in
Your duchess and my husband; one of them
A pickaxe bore, th' other a rusty spade,
And in rough terms they 'gan to challenge me
About this yew. (1.2).
It seems like Vittoria's freaky dream inspires Brachiano to murder Camillo and Isabella. She asks for "protection" from them. Vittoria casts them as the threat, when, in reality, they are sheep to the slaughter—no match for her and the Duke. When Flamineo says that the "devil" was in this dream, Vittoria and Brachiano seem to think he means that the devil was behind Camillo and Isabella threatening Vittoria. But Flamineo is savvier than that: he means that it's a demonically inspired way of getting the Duke to kill his wife and Vittoria's husband (even though, of course, Flamineo is totally on the devil's side.)
Also, the yew tree is commonly found in graveyards. It's a typical symbol of sadness—for instance, Monticelso compares Lodovico to "a black and melancholic yew tree," feeding off of the corpses that lie under its roots (4.3). So its location in the cemetery in Vittoria's dream is particularly appropriate.