Vietnamese grocery store
I know you are looking in the outer boards, but there is a Viet grocery on Laffayette below Grand on the west side of the street. some produce, tons of packaged/canned/frozen stuff.
I go there for Ca Phe Sua packets (instant 3-in-one vietnamese coffee), frozen banana leaves and fried rolls.
Sunset Park Brooklyn . . .
Not exclusively Vietnamese, but lots of the groceries are pan-Asian in their product line, and the Vietnamese sandwich shops all carry some stuff, like tamarind.
Hong Kong Market on 8th Ave at 62nd St has a big selection.
Ditto the Manhattan Chinatown stores, and I'm sure many in QNS.
recommend Hong Kong Supermarket, in sunset park, as above. Look in the back left corner of the store for some specifically vietnamese herbs and for a freezer with the meats also some nearby aisles and end racks have the roasted rice powder,various fermented fish products, etc. Some of the smaller groceries also carry stuff with a Vietnamese focus but its more spotty.