Question

Why do we call a person doing carpentry as engaged in a vacation??

Answer 1

because they are seasonal workers and vacations are in summer.. that's why they were engaged

Answer 2

The word 'vocation' means a strong desire to spend your life doing a certain kind of work.People chose to be carpenters because they have a passion in carpentry and that's why we call the carpenters are engaged in a vocation.As simple as that.
 Hope it helps.