For authenticating documents, the Department of Foreign Affairs at 6 Alexandre de Rhodes St. in District 1 is the place to go.
I liked transacting with the people here. And the fact that it doesn’t get too crowded and the signs are in English. The transaction area is in a well-airconditioned room.
Oh and the best part? Authentication only takes 1 day at the very affordable price of 30,000 VND per document. You don’t even have to pay for it upfront. You just pay when you’re going to get the document already.