Aah, the chord vii: diminshed vs mb5 issue :D
It is kinda related to the dim vs dim7 issue.
Generally, if you extend a dim chord, you get dim7, and if you extend mb5 (i.e. minor w/ b5) you get m7b5.
Let's stick to C major for examples.
Bdim7 = B D F Ab
Bm7b5 = B D F A
Now - chord vii, you...