I'm with the parent comment, as I think the context is important here.
If you think the emergency service vehicles violate the rule, how about a park maintenance vehicle or a park ranger vehicle? Would you say "no vehicles in the park" rule applies to them too, so they would be violating it?
The whole exercise specifically asked us to ignore context, "common sense", etc. If we were supposed to consider an exception for park services, it would have been explicitly stated.
Of all the forums for an insistence on rigidly, literally following rules to make people's heads explode, HN is not exactly the most surprising, but it is one of the funniest.
To me that's precisely the point: since vehicle itself is not defined and is a subject to interpretation, excluding emergency services is less of a problem than carving out an accurate boundary around what is or is not a vehicle.
If you think the emergency service vehicles violate the rule, how about a park maintenance vehicle or a park ranger vehicle? Would you say "no vehicles in the park" rule applies to them too, so they would be violating it?