What is the meaning and significance of virtue and vice in Romeo and Juliet?

Virtue and vice are contraries that are mentioned in Friar Laurence's soliloquy of Act II, Scene 3 in which he employs the rhetorical device of antithesis as he mentions morning and night, darkness and the sun, day and night, good and vile qualities, and virtue and vice, which become a trope for the remaining action of the play. 

In his soliloquy Friar Lawrence contemplates how good can come of "vile," when it is applied in actions for a good end, and how good can become evil if it is put to the wrong use.

For naught so vile that on the earth doth live
But to the earth some special good doth give.
Nor aught so good but, strained from that fair use
Revolts from true birth, stumbling on abuse.
Virtue itself turns vice, being misapplied,
And vice sometime by action dignified. (2.3.17-22)

For instance, in Act III Romeo attempts to stop the heated quarrel of Mercutio and Tybalt by telling Tybalt that he has no quarrel with him, and that he loves him now. But, an angered Tybalt attacks Mercutio underneath the arm of Romeo that is outstretched in friendship and love. Then, Romeo misapplies his good intentions by retaliating against Tybalt, and they, then, become evil.

The final example comes at the end of the play, as the tragic deaths of the violent love of Romeo and Juliet do serve some good as the Montagues and Capulets do end their feud.