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: 
“蛮" usually used in southern Jiangsu province and northern part of Jiangsu near Yangtze river,and Shanghai,but since Nanjing had been the capital in last century,this usage has slided into Mandarin,but people grown up in northern china do prefer "很" to “蛮”.
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.