A covenant, and a contract in marriage, or in a rental agreement, or a car purchase, a mortgage all require a commitment on your part to repay. A contract between you and your painter require an agreement in writing on what will be done and what will be paid etc.
Thinking that Jesus ruled out all these things is simply a misunderstanding about what he was talking about. But you will figure it out as you keep reading and learning. I think you are getting hung up on the English word "Vow" and because that word is used in marriage.
When you try to contend for a particular word in English you must consider that word in Greek first. When someone says "I swear to God" or "I swear by the temple of God" Jesus said don't do this. And this has nothing to do with marriage contracts.
Swearing by God in court always seem strange to me. Seems like that is the kind of thing Jesus said not to do.
I am not going to argue with you about it. I am here to gently and patiently help out. In a format like this it is hard sometimes to know if people are open to learning or if they are suspect, and afraid to listen, or if they are just not understanding. So I will assume the best and leave you to figure it out. Stay humble and always learning. Good luck to you.