It's been really difficult for us to find the place and to find the right way so we hope it helps for the next travellers ;)
Vietnam Consulate is located in Hongta Mansion inside Kai Wah International Hotel, 5th floor. Beijing Road, 155. (easy to take a bus to arrive there, close to Train Station).
If you look for the info in their website it's old and nobody in Kunming helped us neither the people from our hostel, it's like a secret ;)
Finally we did it in one day and pay more but it's worth for us avoiding more nights in Kunming.




