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?
As others have said, 蛮 is not exactly dialectal. Most Chinese speakers would understand what you mean if you use it to mean "quite."
However, it should be noted that it is colloquial--it shows up in spoken Chinese but rarely in print, hence your discovery of it on a Chinese TV show. In fact, some native Chinese speakers will confuse 蛮 with 满 when asked to write the word down.
“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.
“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.