A vow is a purely religious act which cannot be taken in a fit of passion. It can be taken only with a mind purified and composed and with God as witness.
A principle is the expression of perfection, and as imperfect beings like us cannot practise perfection, we devise every moment limits of its compromise in practice.
A man who was completely innocent, offered himself as a sacrifice for the good of others, including his enemies, and became the ransom of the world. It was a perfect act.