Here ‘vessels’ are metal receptacles such as buckets, oil-drums, and petrol-cans, which, when struck, make more noise when they are empty then when they are full. The meaning if the proverb is that empty-headed persons are always the most talkative and noisy. In The Deserted Village Oliver Goldsmith spoke of:
The noisy geese that gabbled o’er the pool,
The playful children just jet loose from school,
The watch-dog’s voice that bayed the whispering wind,
And the loud laugh that spoke the vacant mind.
A similar proverb is They brag most who can do least. The Dutch have a saying that one penny in a money-box makes more noise than all the coins in it when it is fool.