Is food useful?
Solution 1:
Food has no effect on vigor or vitality.
You cannot even use (eat) it.
It can only be traded or dropped like any other junk item.
Solution 2:
Food is listed in the "Junk" category, which means, if it is not also a quest item, it has no use and can be safely sold.
Solution 3:
There might be some NPC's that ask for food before giving you more information. I think I met one in Flotsom...