use List::MoreUtils qw/natatime/;

my @unique_vault_list = 0..31;
my $iter = natatime 20, @unique_vault_list;

while (my @cur = $iter->()) {
  ## now you have each group of 20 elements in @cur
  print "@cur\n";
}
__END__
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19
20 21 22 23 24 25 26 27 28 29 30 31