I need to omit chicken stock from a potato gratin recipe. Do I increase the cream, substitute with veg stock, or is it OK to just omit?

I agree with Big G-man, most potato gratin recipes do not call for stock. I would increase your cream or you could use a combination of cream and milk. Check out this recipe that only uses milk http://food52.com/recipes/11595-pommes-dauphinoise-potatoes-au-gratin.

replace with vegetable stock

you're going to need the liquid in the gratin, no matter what it is.

I have never made a Potato Gratin with stock ... always just heavy cream