How important is the energy to manufacture vehicles?

Not as important as the fuel they use.

This is analyzed here ( http://www.scientificamerican.com/article.cfm?id=green-is-a-mirage ). It's an interesting article.

Here's the relevant quote for EVs:

"An LCA reveals that in terms of global warming effluents, for example, everything in the car's life cycle from manufacture to getting scrapped pales when compared to the emissions while it is driven."

So, it really is the fuel used for driving that matters.