Combat zones, however, indeed don't seem to be that well-defined. It says that in a zone you only need a few steps to meet someone in melee combat, which to me suggest between 0 and maybe 15 feet. It's not clear if this is from one edge of a zone to the opposing edge. If so, that would make "3 zones away" no more than 45 feet away, which seems rather underwhelming when you're firing rifles or crossbows.
I think that you are giving the distance to much attention. The size of a zone is not only dependent of the distance but also dependent of the terrain. The Vaesen rule system is rather narrative and abstract in design. I think it is not of any interest if you are standing at the edge of a zone or at the opposing edge of that zone. The question is whatever the characters are in the the long open grass field, at the beach or in the boat or maybe in the water which is between the beach and the boat. Two characters standing in the boat are in zone 0 to each other while a character standing on the beach and a character in the boat are 2 zones away from each other (beach <-> water <-> boat). A character that has withdraw in panic on the open grass field may be more than 3 zones away from the beach. Or only 1 zone away from the beach if the character has been moving towards the beach but hasn't really got there before the combat begins. You don't need a grid. You just have to keep the zones in mind or draw some areas on a blank paper and name the areas grass field, beach, water, boat etcetera.
Please correct me if I'm wrong!
UPDATE: I think zones in Vaesen is supposed to work in a similar way as zones do work in FATE. Take a look:
https://www.youtube.com/watch?v=70njl8wahRA&t=1015s