Question: I was watching a Chinese show today, where they said '蛮舒服'. Since I've never heard 蛮 before I looked it up, and the dictionary tells me that 蛮 is only used as 'quite' in dialect. Now my question is, where?

Answer: 
I lived in 江苏 and 上海. I used it in preference to 很 (I also used 好 in preference to 很). I certainly don't know the total geographical reach, but it's very common at least in the areas I was living.
“蛮舒服” means “比较舒服”, 蛮 expresses some extent like “还可以”（It is fine/OK.） It does not saying very "舒服", or you are just expressing some feeling that you feel comfortable. You don't want to stress that it is comfortable very much.