If your feathers are a little ruffled just put them down this is not to get people all upset. This is a choice that people have. The first thing that comes to everyone's mind is "SATAN worshippers." Don't judge it is not the case at all. I have done some weddings that have asked for this type of vow and they are all good people. In some cases nicer people. The couples were not Christian but of different faiths and wanted a nice ceremony. The vows I use are heart felt, warm and speak more about God without the mention of the name. I incorporate those vows in with vows that mention God. They are also used by people of different faiths getting married to avoid fighting among the families. That is more of a problem.
All that I have to say is, keep an open mind and listen to the words. God always speaks to those that are listening.
Blessings on your day!