What is the main theme of "The Nun's Priest's Tale" in The Canterbury Tales ? Discuss it with examples from the text.

The main theme of the Nun's Priest's Tale, I would argue, is vanity, especially as it is related to the dangers of flattery. Chauntecleer, a large rooster, has a terrible dream one night in which he's threatened in the farmyard by a strange orange beast. One of this many hen-wives, Pertelote, implores him not to worry about it. It was just a dream, most probably brought on by something he ate. What he needs is some good, strong laxative.
But Chauntecleer's not interested. Dreams always have some meaning or other; he has this on the authority of none other than the great Cato, the famed Roman statesman and philosopher:

"Madame," quod he, "graunt mercy of youre loore. But nathelees, as touchyng daun Catoun, That hath of wysdom swich a greet renoun, Though that he bad no dremes for to drede, By God, men may in olde bookes rede Of many a man moore of auctorite Than evere Caton was, so moot I thee, That al the revers seyn of this sentence, And han wel founden by experience That dremes been significaciouns/As wel of joye as of tribulaciouns That folk enduren in this lif present.

In modern English, what Chauntecleer's saying is that according to Cato dreams always signify something, even though we shouldn't be frightened of them. Although he says he's grateful for Pertelote's offer to help him with medicine, he rather patronizingly dismisses her homespun remedies. Here, Chauntecleer is displaying intellectual vanity, privileging ancient book-learning over traditional folk medicine.
Nevertheless, Chauntecleer becomes reconciled to Pertelote and agrees to renounce the vision contained in his dream. However, one form of vanity is replaced by another. For later on, Chauntecleer is approached by a wily old fox who's been observing him and his hens, lurking among the cabbages ready to make his move. No prizes for guessing what the fox has in mind. Chauntecleer has a beautiful voice which he loves to show off by singing. The fox sees his chance and after emerging from the cabbage patch starts shamelessly flattering the proud cockerel and asking to hear his singing voice:

For trewely, ye have as myrie a stevene As any aungel hath that is in hevene.
(You really do have as merry a voice as any angel in heaven).

And on it goes until Chauntecleer, so ravished by the fox's flattery, and beating his wings with pride, stands on his toes, stretches his neck, closes his eyes, and begins to crow loudly. The fox sees his chance and pounces immediately. He grabs the poor rooster by the neck and runs away, carrying the hapless Chauntecleer on his back.
But Chauntecleer has the last laugh. He takes a leaf out of the fox's book and starts to play on his vanity. He advises the fox to brag about his taking of Chauntecleer to his pursuers, the farmyard animals and dogs who've given chase. And for good measure, he might like to curse them:

Turneth agayn, ye proude cherles alle! A verray pestilence upon yow falle! Now I am come unto the wodes syde; Maugree youre heed, the cok shal heere abyde. I wol hym ete, in feith, and that anon!
(Get lost, you peasants! A plague upon you all! Now I've reached the woods the cockerel's going to stay right here despite all your best efforts. And I'm going to eat him right away!)

The fox foolishly agrees, and opens his mouth, ready to let rip with some medieval smack-talk. Suddenly, Chauntecleer flies out of the fox's greedy mouth to the safety of a tall tree. The rooster's learned his lesson as he tells the now hungry, frustrated fox:

Thou shalt namoore thurgh thy flaterye/ Do me to synge and wynke with myn ye; For he that wynketh, whan he sholde see, Al wilfully, God lat him nevere thee!
(You won't get me to sing again through flattery. For if you close your eyes when you really should be seeing, then God will never let you prosper).