My current tr/// (/d still not doing what I expect it to):

$primary =~
tr(aàảãáạăằẳẵắặâầẩẫấậbcdđeèẻẽéẹêềểễếệfghiìỉĩíịjklmnoòỏõóọôồổỗốộơờởỡớợpqrstuùủũúụưừửữứựvwxyỳỷỹýỵz)
  (0000001111112222223456777777888888abcddddddefghijjjjjjkkkkkkllllllmnopqrrrrrrsssssstuvwwwwwwx)d;

$secondary =~ tr#aàảãáạăằẳẵắặâầẩẫấậbcdđeèẻẽéẹêềểễếệfghiìỉĩíịjklmnoòỏõóọôồổỗốộơờởỡớợpqrstuùủũúụưừửữứựvwxyỳỷỹýỵz#\x00-\x5d#d;