People have their vices and some are worse than others. One reason out of many not to believe Medjugorje is that the BVM is said to have first appeared to the kids while they were smoking cigarettes out behind a shed. I don't believe the BVM's going to appear to anybody smoking cigarettes, and that's way before weed. I think smoking weed's another vice and worse than alcohol or tobacco.
Why not? In and of itself, smoking tobacco is no sin.
I was sneaking an illicit cigarette in my high school boys' restroom when I got to talking to another student (who did not smoke) about becoming a Catholic, he said "so why don't you?", and I called the priest that afternoon. (I didn't mention the smoking.)
I was received into the Church a few months later, baptized and confirmed.
Who can say what I would have done if I hadn't been in that restroom, smoking that cigarette, and getting in that conversation? (To be fair, it had been on my mind for some time.)