I just played my first game yesterday and had the exact same question.
Now I re-read the rules and think:
Building a "vehicle" does not include plows or peat boats.
It says in the rule book, page 14:
There are 6 different types of Vehicles. They have different costs (and different Victory Point values – VP):
- Handcart (0 VP)
- Wagon (1 VP)
- Cart (1 VP)
- Horse Cart (2 VP)
- Carriage (4 VP)
- Droshky (5 VP)
Point Number 3 is cited in the spots which speak about "building a vehicle". So it suggests that you can neither build a plow nor a peat boat unless the action allows you to build one. Rather than saying that you can build a vehicle.
This point is clarified once again on page 11:
Plows and Peat Boats are not considered
Vehicles, although – like Vehicles – they must be kept in the Barn.