Thief & wagons

The thief can stole a wagon?

By the definition of the rule, the thief can do it because a wagon is a common object.

But, a friend says that is incorrect because the wagon is a vehicle and not and object.

Although it does push credibility beyond where the rules intend it to go, by definition he can steal a wagon, as you say it is a common item. However in your friend's place I would point out that he cannot steal a Horse or a Mule etc as they are all Rare items.
So he can assume right of ownership of the wagon but he is going to have a tough job hauling it around the city manually.

But when you buy a wagon, it already comes with the horses, right?

In my games I say not, they have to be bought separately unless you can justify a horse and cart or wagon or coach in your starting equipment (Carnival of Chaos for example). Up to your game group to determine for your table.

But either way he is not buying it, he is stealing it and he cannot steal the horse/mule/burro/Rhosgobel Rabbit etc. Have to assume they run off when he tries to coerce them into coming with him.