Ai ơi đừng chóng, chớ chầy, Có công mài sắt, có ngày nên kim.

O, do not rush, do not delay, those who put in effort to grind iron will one day have a needle. 

The meaning is hard work will be rewarded (i.e., grinding iron until it becomes a thin needle). This is equivalent to “where there is a will there is a way” and “practice makes perfect.”