A blog on the topic of tailors in various cities in Vietnam, says that for Hanoi:
As a recommendation to make a suit to measure in Hanoi, the best
tailor and the best store is Cao Minh. Cao Minh also tailoring shop in
Hanoi, also owns two shops in Ho Chi Minh City. Cao Minh Hanoi is
located at 250 Hang Bong Street (a few blocks south of the lake of
Hanoi). Cao Minh also has online shop, but do not buy a tailor-made
suit online unless they have made us one previously and we have the
exact measurements.
A lengthy review on the topic on TripAdvisor suggests:
Bambou Silk
8 Nha Chung
It's in the Old Quarter, near the cathedral.
¬¬¬¬¬¬
Another shop he got a good suit from (also in the Old Quarter) was:
Duc Minh
24 Hang Da
(I've trimmed snippets out of the lengthy review, for more detail click the link and have a read - they provide good descriptions of both)