Does vitality regeneration stack?
Well, Yes and No.
There are 3 different types of "food": Eatable, Drinkable and Potion. These stack with each other. So you can drink Cow Milk and eat Nuts and these effects will stack. If you eat 1 Nut and then 1 raw meat, the remaining healing effect of the nut will vanish. Otherwise it would be way to easy since you could eat 20 Nuts and were unkillable for 5 seconds.