these are all decidedly toward the southern end of your range, but i like these three viet places. pho grand has maybe the 'nicest' atmosphere -- feels a little more spacious and less clattery than the other two, which are more standard chinatown-esque storefronts (though nam son is much bigger than pho bang). all are casual, cheap, and (relatively) fast (i've had the best service at pho bang -- they can be a little slow at pho grand).
pho bang @ 157 mott
pho grand @ 277 grand
nam son @ 245 grand
245 Grand St, New York, NY 10002
157 Mott St, New York, NY 10013
277 Grand St, New York, NY 10002