I'm an international student from Chile, currently I hold a F1 visa ( studying in Portland ,Oregon) due to end of term and graduation this December 15th 2013 my F1 will expired. I hold another visa B1/B2 tourist that isn't valid in the US yet ( since I entered in the US with a F1 visa , this B2 doesn't have the stamp from customs ). Would it be valid if I go to the boarder between Washington state and Vancouver Canada ( but never entering to Canada because I don't have a Canadian visa) and validate my B2 visa before the F1 expires without me having to go back to Chile and then come back to Portland? Would I be able to re-enter to the US from the boarder with no problems?

F-1 holders are allowed to have 60 days after the date on D/S for packing/moving/returning home.

If 60 days term is not enough then you will have to cross the border and return. Not 100% confident that immigration will allow you to come back, it's best to bring all supported documents.