Why do Hindus worship vehicles on the day of Dussehra (vijaya dasami)?
It's just a custom to show respect for the technology that has bettered our lives. Hindus of different trades honor their respective tools and implements (a soldier might, for example, honor his weapons).
I think that the Wikipedia article sufficiently explains the origins of the custom. Read the first section under the Wikipedia article ("Legend") which explains the two chief mythological and legendary reasons for this puja.