If you really want to taste good delicious Vietnamese food you'll have to visit Bodhi Chay. Most of the Asian snack bars and restaurants in Hannover only have asian meals that are made for the tongues of Europeans. At the Bodhi Chay there a lot of interesting meals where you can discover the kitchen of Vietnam.
Only a few dishes are not vegan.
The staff is friendly, but the older man there doesn't speak good German. So sometimes it could happen that you get something what you didn't ordered. Better order at the woman. ;-)
You can have your meal for take away or you can eat it there. Nice atmosphere there. Often Vietnamese music, the interior is Buddhistic.
Prices are cheap. :)
Enjoy your meal! :)
Cons: Only cash