For starters, don't use \@item, use [ @item ]. That'll get rid of the weird junk at the end. Then you can flatten the vvalue return as follows:

vvalue : primitive vector_sep(?) vvalue
         { [ $item[1], @{$item[3]} ] }
       | primitive vector_sep(?)
         { [ $item[1] ] }
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Your grammar can be optimized to remove unneeded backtracking. Here's a (completely) equivalent grammar:

vvalue  : vvalue_(s)
          { [ map { @$_ } @{$item[1]} ] }
vvalue_ : primitive vector_sep(?)
          { [ $item[1] ] }
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Both return:

$result = [
            '2',
            '3',
            '5',
            '7',
            '11',
            '13',
            '17'
          ];
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