I just got my Dahon Vitesse and it looks great. Will give a rewiew in a few days but first can anyone tell me what this is for? It came in the box and I don't know if I need to put it on the bike before I can safely ride it?

It is a derailleur hanger in case you ever want to replace the custom "Neos" derailleur with a standard one.
You do not need to attach it to the bicycle in order to safely ride it.

I hope however that you have had the bicycle looked over by a competent bike mechanic ... if you just received the bike in a box from some internet shop, then there is the potential it might need some adjusting - even if it was adjusted prior to shipping by a good dealer, being jostled about in a UPS truck might loosen something etc.

Is there a reason why I would want to replace it?

Thanks

no reason to rplace the neos derrailleur .. itsd a good piece of engineering and works well....

The reason the derrailleur hanger is with the bike is the constant bitichin from some folks who think that Dahon has too many propietry parts. ( Which is not true compared to othe folding bikes )
But the Neos derrailleur is indeed a special Dahon part and might be difficult to find during your next world travels. With the replaceable derrailleur hanger you can "repair"it with ANY derrailleur you like.

Thor