Trâu ác thì vạc sừng.

Evil buffalos have their horns chopped.

The meaning is someone who is evil will be punished. This is equivalent to “Someone getting what they deserved” and “Just deserts.”

Ăn mận trả đào.

Eat a plum, repay a peach.

The meaning is repaying kindness with a better gift as in “Ăn mận trả đào, ăn quả trả vàng / Eat a plum repay a peach, eat a fruit repay with gold” (i.e., peach is more valuable than plum, gold more valuable than fruit). This is similar to “Do unto others as you would have them do unto you.”