Siege weapons, in terms of the player's interaction with them are devices, mechanical or otherwise, that can act as impediments to the further progress of an adversary.
They would not get treated as items in
4e, nor as vehicles.
The question then to ask is: For this encounter, is the siege engine there as backdrop, or as a direct threat to the creatures themselves.?
If it's a backdrop, then it's considered terrain at most. For example, a siege tower doesn't need complicated rules for 'It's a way to get from here to here.' You can assign it defenses and hps if you feel it is appropriate. Otherwise, it gets the same treatment as an overcomplicated ladder.
If it's a direct threat/impediment to the creatures themselves, then simply stat it up exactly as you would any other trap. It has a trigger, attacks, consequences of hitting, which it would apply to unfortunate creatures in its path.
If it's neither of those things, it's just a plot device, and there has never been a need for rules for plot devices.
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