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”.
“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.