Well, true that Google maps are good.. but you spend data on that one. you could get HERE maps completely free and you can use it in Ofline.. plus it has turn by turn what google maps doesn't got in Vietnam. https://www.here.com
If you are gonna tour then you might want to get a copy of giao thong duong bo vietnam . it's a road atlas of the whole country.
I have a nice scan of the northern part of the country that I can email you.
if you want more details and ordnance survey type maps then there are places to get them, but they are specialist, do not cover large areas and a little pricey.
You can look this..
there is a map to bookstore, but that bookstore (well, at least very big.) is one cross road up and little bit towards right.. you should find it easy. I have one too, but I think only one time did carry it with me, my mate did use it over a week when I lend him a "scooter".