vla-item is used on collections, not lists.
IF, and that's a big IF, ListA is a true list of AttributeReference objects and RefAtt is an AttributeReference object then perhaps this ...
(setq result
(
(lambda ( / tagstring )
(setq tagstring
(strcase
(vla-get-tagstring
RefAtt
)
)
)
(vl-remove-if-not
'(lambda (attributeReference)
(eq tagstring
(strcase
(vla-get-tagstring
attributeReference
)
)
)
)
ListA
)
)
)
)
Coded blind, so may need massaging.