A combe is usually at the top of a mountain. I feel un vallon is probably closer to a hollow, which can be found in a plain or within a valley. There’s also une dépression and (more colloquially) un creux (litteral equivalent of a hollow) but I feel they refer more to the shape of land than to the location. So you could say

Il y a une dépression derrière le village

Il y a un vallon derrière le village

Je vais au vallon derrière le village

but not

(*) Je vais à la dépression derrière le village