It's not Turnip or turnips, the quiz setter has confirmed this and it is 8 letters.
It is plural so if we take s off the end it's 7 letters and the additional clue is that whatever the 'vessel' is, it is physically found inside the fruit (or vegetable)
thanks for the links and the radishes suggestion but the vessel, (I guess your meaning 'dish' in this answer) is not found that way, it's physically something found with-in the fruit or veg that is the answer, something like, stone, seed, pip... actually inside the fruit or veg that is the answer.
Sorry, I think we were all thinking the same re vessel inside the name rather than the fruit itself. I'll keep thinking but suggest you don't hold your breath.