That depends on what language we're speaking of. Obviously, in English, "Christian marriage" vowels would be i, a, and e. This is true even when using colloquialisms like "getting hitched." Those preferring terms like "holy matrimony" have to account for the vowel o, and should be ready for the inevitable debate about whether or not y counts as well.
For brides who love cutesy kitsch and don't care how silly it looks, there are other options. For instance, one could replace all of the "I" vowels in the programs with little stick figures of Jesus, the "o" vowels with halos, and so forth. Opting for "Christian marriage vowels" is a great way to look like one of those creepily-enthusiastic Disney nuts, just swapping Mickey Mouse and Jesus Christ. If your wedding theme absolutely must scream "nominal Christian," this is a great option.
What's even more concerning is modern Christians' neglect of the wedding consonants. Many brides every year are horrified to arrive at their receptions to realize some people are there unexpectedly while others who promised to come didn't show up. All because friends and family ignored the letters R, S, V, and P.
If you go beyond titles and invitations and include the
marriage vows, you're probably going to cover all the vowels, including "y." In languages other than English, the concept of a Christian marriage vowel is less clear. If you're planning to be married in Hong Kong or Beijing. . .good luck.