Ăn ngay nói thật.

Eating straight speaking truth.

The meaning is be honest. This follows the pattern of combining the 2nd and 4th words together for the meaning (ngay thật / straight truth). This comes from “Ăn ngay nói thật, mọi tật mọi lành / Eating straight speaking truth, all bad all good” (i.e., be honest about your strengths and weaknesses). This is equivalent to “Open and aboveboard” (i.e., aboveboard refers to hands above the playing table).

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.”