Someone posted this on TripAdvisor last April 2018 ...
"After 4 different hospitals/vaccination centers, I finally found Stamaril (the latest yellow fever vaccine) at the Center for Preventive Medicine Hanoi, located at:
70 Nguyễn Chí Thanh, Láng Thượng, Đống Đa, Hà Nội
The staff did not speak any english, so have google translate ready. You will have to go back and forth between 2 different buildings. One building is where the office to sign up for the vaccination is. Then you take a form to the actual vaccination payment office in a second building. Make the payment (590K VD at the time of this post) and take your receipt across the hall to receive the injection. Once we got the first form and figured out what was going on, the whole process only took about 30 min."
Their website is: http://www.ytdphanoi.gov.vn/
Điện thoại: (0243) 834 3537
(the TripAdvisor post was June 2018)