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: 
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.
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.