What are Macduff's responses to Malcolm's vices?
The exchange between MacDuff and Malcolm serves for Shakespeare to examine the difference between a tyrannical ruler and a noble/just ruler. Malcolm presents many different terrible traits, many of which he compares with the tyranny that Macbeth has presented, in order to demonstrate what negative impact tyrannical rule has on a country. However, for the purpose of the play Malcolm is truly just presenting this image of the tyrannical ruler to test MacDuff's loyalty to Scotland and ensure that he has not been sent by Macbeth to lure MacDuff into his demise. When MacDuff hears the full extent to which Malcolm claims to be a tyrant, he acknowledges that Malcolm should not be allowed to rule if he is truly the tyrant he claims to be. This shows incredible honesty and loyalty to Scotland because he is willing to openly tell Malcolm that he is not fit to rule if he is a tyrant like Macbeth. This in turn buys Malcolm's trust and he enlists MacDuff's help in defeating Macbeth. From a thematic perspective, Shakespeare uses this exchange to develop the message that tyrannical rule of any form cannot be best for a country and must be overthrown by a just and noble ruler who possesses many of the virtuous traits that MacDuff exemplifies.
I'm making the assumption that you're referring to Act IV, scene 3. At this stage in Macbeth, Malcolm is trying to test Macduff's loyalty. Malcolm hopes that he can find out exactly how Macduff feels about Macbeth by pretending to be an even bigger tyrant than Macbeth. Macduff falls for it and shows Malcolm his true feelings regarding Macbeth. In his test of Macduff, Malcolm pretends that he will will be an even worse leader:
In Act 4, Scene 3, lines 58-67, Malcolm claims that his sexual desires are insatiable and that no woman in the country would be safe from his lust. "But there’s no bottom, none, In my voluptuousness. Your wives, your daughters, Your matrons, and your maids could not fill up The cistern of my lust, and my desire All continent impediments would o'erbear That did oppose my will. Better Macbeth Than such an one to reign."
Macduff responds by saying that if Malcolm is the rightful heir, his lust could be satisfied in secret. He indicates that many women would be happy to service their king and that Malcolm could still appear virtuous. (4,3, 70-75)
Malcolm then follows up with his second vice, that of greed. He states that he would take from his nobles and grow greedier with every stolen item, eventually destroying them in the process. (4,3, 80-86)
Macduff counters with the fact that Scotland is rich enough to satisfy Malcolm's greed, although he admits that this particular vice is much harder to conceal. He also believes that Malcolm's good qualities should outweigh his bad ones. (4,3,86-92)
In a final test for Macduff, Malcolm states that he has no good qualities. He flatly says that he doesn't possess any of the traits that a king should have, "As justice, verity, temperance, stableness, Bounty, perseverance, mercy, lowliness, Devotion, patience, courage, fortitude." He even indicates that he would end any semblance of peace found in Scotland, forever damning it to havoc and rapine. (4,3, 93-103)
Macduff begins to weep for Scotland and her fate at this point, since Malcolm appears to be just as bad for his home and country as Macbeth. His hopes for the future dashed, he begins to mourn for his country and her future. He wonders how someone like King Duncan could have had such a horrendous person like Malcolm as his heir. (4,3, 104-117)
Malcolm comforts Macduff, realizing that Macduff is loyal to Scotland and her future, enlisting him to his fight against Macbeth.