as far as im aware it should be 2 years warranty on your nokia handset. Doesn't matter what network it is.
Altho issues can arise if it is branded firmware and then u de-brand it with generic firmware as networks don't like that.
Overall if anything happens and vodafone give u hassle? then just send it over to nokia and they should fix it for you without any questions.
Although i dont think or know if you can change the product code on the n900 or if it even has 1?
another thing to note is that i had a nokia n95 8gb on the 02 contract which i debranded etc, i had an issue with it an sent it to nokia they put the 02 firmware back on as i changed the product code back to default. This was mid last year when this happened.