Anh hùng gì? Anh hùng rơm, Tôi cho nắm lửa hết cơn anh hùng.

What hero? Just a straw hero, I will let him hold fire for this hero fever to pass.

This is equivalent to “barking dogs never bite” (i.e., someone who talks big but is a coward). A bundle of straw burns intensely but quickly disintegrates.

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.”