Áo người mặc đoạn cởi ra, Chồng người ấp mượn canh ba lại hoàn.

Dress of another is worn for a bit and taken off, A borrowed husband from the hamlet returns at the third night-watch.

The meaning can be 1) similar to “you cannot lose what you never had”, i.e. borrowed things do not last and 2) do not be a homewrecker/the other woman.

Ai đem con sáo sang sông, Để cho con sáo sổ lồng nó bay?

Who brings a starling across the river, To let it grow feathers and fly away?

The meaning can be 1) being sad when the one you love goes to marry someone else or 2) being disappointed when all your efforts/feelings have been in vain similar to “all that work, down the drain”. Starlings are birds that are raised as pets and can be trained to talk. Going across the river refers to going to marry.