South American of pure and vain disposition (8)
I believe the answer is:
peruvian
'south american' is the definition.
(someone from Peru)
'pure and vain disposition' is the wordplay.
'and' means one lot of letters go next to another.
'disposition' indicates an anagram.
'pure' after 'vain' is 'vainpure'.
'vainpure' with letters rearranged gives 'PERUVIAN'.
'of' is the link.
(Other definitions for peruvian that I've seen before include "Certain South American" , "Eg, native of Lima" , "Nationality of citizen of Lima" , "Paddington's nationality" , "of South America" .)