No there is no tutorial, basically you have to know, php, html, css, and have a understanding of how vBulletin works. Then take it and put it all together.
Best way to learn is download a mod, see how the developer did what they did.
I would start on something small, that requires no DB manipulation, because if you mess something up, you can screw up peoples DB beyond repair if you don't know what you are doing.
Then once you think the mod is ready, test, test, test. I extensively test my mods, before I release them. Most bugs are caught before the mod reaches the users hands. But sometimes they still slip through.