I solved that problem a little differently:

: reverse-vowels-2 ( str -- str' )
  [ clone ] [
    >lower [ vowel? ] find-all
    [ values reverse ] [ keys ] bi
  ] bi                       ! str vowels idxs
  [                          ! str | vowel idx
    pick dupd nth            ! str | vowel idx orig
    1string upper?           ! str | vowel idx t/f
    swapd [ ch>upper ] when  ! str | idx vowel
    set-nth-of               ! str'
  ] 2each                    ! str'
;

Maybe clearer on lemmy without the comments:

: reverse-vowels-2 ( str -- str' )
  [ clone ] [
    >lower [ vowel? ] find-all
    [ values reverse ] [ keys ] bi
  ] bi                       
  [                          
    pick dupd nth            
    1string upper?           
    swapd [ ch>upper ] when  
    set-nth-of               
  ] 2each                    
;