Don't remember the notary, but there are a bunch of them, it was a couple blocks from the licensing office. Make sure you tell them you need the motorcycle portion converted, they only included the car on the first try, Had to do a mad scramble to run back to the notary and have them fix it on the spot, then run back to the licensing office just before closing. I have heard of others having the same. I went to the licensing office at 252 Ly Chinh Thang in Saigon, which was recommended on multiple sites.
vietnamcoracle.com has a bunch of detailed information for routes and where to go. Tigit and Saigon motorcycles are two reputable sellers/renters
If you don't know already, do not under any circumstances buy a Honda Win. The only people in Vietnam that have one are stupid backpackers.