Names are written horizontally, corresponding to the current habit of reading horizontally from left to right. In modern life and work, signatures are mostly written horizontally, such as check signature, document signature, contract signature and so on. Vertical writing corresponds to the ancient habit of reading vertically from right to left. In daily life, when the custom is traditionally signed, most people write vertically, such as paintings and calligraphy, red envelopes and so on.
Signature is an art and reputation. Generally, you should write a greeting message and your name on the back of a red envelope, because there will be many relatives and friends on the wedding day, and you can't remember who sent it unless you write your name horizontally. Of course, if you are close, you'd better tell a few jokes to enliven the atmosphere and make friends happy that you will attend his wedding. So it is ok to write the name vertically.