I think the word vocabulary should be used as a countable noun in the above situation, because it was not used to refer to a collective concept, but quite specifically some sets of words like ferocious, scrawl, etc..
This is just my personal opinion. Any other thought/ explanation is welcomed.
Elevens2018-07-20 16:26:37
But ielts is the major English examination widely recognized by different large-scale organizations all over the world