Merry, you mistake some cultural trappings for a religion. I'm an atheist and I celebrate Christmas. It's a tradition but it doesn't have religious meaning for me. I also celebrate Midsummer's Eve (as does everyone in Finland) and it's an old pagan tradition, origin's of which have pretty much been forgotten. Cultures today constantly refer to things about past religions and mythologies that are no longer believed to be actually true.
And Vulcan priests are people who are familiar with Vulcan traditions. There are no Vulcan gods.
And why is praying illogical? Why would you even ask that? It is either supernatural thinking (if you think it actually does something) or a mere gesture (if you don't). Now latter is really not a problem in human sense; saying 'I pray your journey will be safe one' may be just saying that I really wish nothing bad happens to you on your journey. It is still kinda odd thing for a Vulcan to say though.
In any case, people in Trek do not talk about spiritual things. Never do Starfleet personnel speculate whether a random super energy being might be an angel, a god or have any other spiritual meaning. Never do they ponder about afterlife or salvation of an alien species. They approach things from a purely scientific viewpoint.