The more I think about the more I believe that this is a mappers' issue. Let the mappers decide what vehicles or not even having vehicles in a givrn map. In addition I assume that a mutator or some option to turn off vehicles completly, maybe on a per map basis.
I would like to see vehicles that are somehow at the discretion of the players. For example. Having boats on pre set on a map with a large body of water to get from point A to point B is nothing special. Kind of like the assault boats in BF 1942. If whether the players had access to boats was at the discretion of the players, and the map had a body of water as one of numrous paths, then the items would feel more like tools rather than part of the maps obstacles.
The beauty of INF is that the weapons feel like tools rather than a Hierarchy of guns. each Weapon is effective in it's proper application. I would like some of the less conspicuous vehicles to provide this same feeling.
as to how to integrate vehicles, like the assault boat, in a fair way as well as giving the player ability to call upon it WHENVER he likes is the tricky part. (laodout system? doubt it. can someone think of soemthing?)