The thing about sapping, mining or vehicle combat is that the caveat that the AC and hit point levels are only if the appropiate tools are used.
e.g. cannon for ship to ship combat or whatever, not hand weapons.
Yes, in general I agree with you, but you then assume that most ships only have ship-to-ship capable weapons equipped up to a certain point. That gets pretty boring -- it assumes a lot of the background fun of combat becomes mere support for the SINGLE piece of equipment/person who can deal damage to a big-ass vehicle.
Let's assume we use the airship scenario I presented earlier:
I established the baseline airship in my campaign to be the RAW airship from AV1. I considered that type of ship to be the standard mercantile ship of the only airship-building nation in the known world. That nation -- let's call it Archaven for expediency -- also had military, spy, and exploration vessels. I established that the 'standard' Archaven airship normally had only one, if any, ballista. The more state of the art ships could have up to three. So there, by those specifications I can establish an entire order of battle for the world (which I did).
1) And so let's say the standard amount of damage one ship could do to another ship was between 1-3 ballista worth.
/unnecessary details ------
From there I went on to establish that the Archaven airships were more alchemical and artifice in nature, and thus could be retro-engineered.
So then, I then established that the Archaven airships have only been out of their R&D/prototype phase for less than twenty years.
In that time only about 25-30 standard/mercantile ships had been built and there were only about 10 more superior designs.
Therefore I could have a military PC knowledgeable in History (i.e. military knowledge)/ or someone outside the Archaven line knowledgeable in Streetwise (i.e. leaked military intelligence) in account for all of the airships in the world up to a certain (difficult) DC.
I also established a single rogue shipyards of an evil nation that was in the process of retro-engineering a few stolen airships.
Skill Challenges or simple skill checks could therefore be used to figure out the association of any given airship spotted (especially if airship was seen up close).
/end unnecessary details.
2) So we establish that only ballistae or some other similar type weapon can do significant damage to an airship (while flying and manned; not talking about a clandestine sabotage, etc.).
But the big question is what the heck do we do about opposing spellcasters? Do they just buff their respective boarding parties and then the object becomes to hover adjacent to another airship while the two crews hack at each other to death (ala Greco-Roman style)?
Or do we allow for the possibility that spellcasters of a certain kind (maybe wizards or ranged strikers -- sorcerers, warlocks) can figure something out?
Those are the ad hoc questions I quickly came to have trouble with during my airship campaign.
I ended up have airships turn more into floating landscape by the end, because I realized that for every PC who could set fire to an airship with Flame Seed or something similar, I could have NPC enemies do the exact same.
Anyway, I know its a corner case, but it made me realize that the 4e vehicle combat rules had problems depending on the 'degree of magnitude' size of the vehicle.
If any of you are still reading this screed, you can see the old website for the campaign in question:
The Spire & Abyss.
C.I.D.