problem... I want to enter Hanoi one day earlier then Visa says

I have the same problem!
Therefore I called the embassy and they informed me, that I have to send them the passport again; they will not charge me any additional costs anymore.

As others have said, Vietnam does not screw around with this and your dates have to be exact. I was on the train from Beijing to Hanoi last year and there were a few folks on the train that had the same type of issue - they got booted off the train in Nanning and the train people would not let them go on towards Dong Dang. Bummer. Get a later flight...

:)