use Unicode::Normalize qw(NFD);

sub is_greek_vocalic($) {
    use charnames "greek";
    die "wrong args" unless @_ == 1;
    local $_ = NFD(lc(shift()));
    s/\pM+//g;            # combining marks from NFD form
    s/\p{Diacritic}+//g;  # eg, GREEK DASIA, which is \p{Sk}
    return scalar m{
        ^
        [\N{alpha}\N{epsilon}\N{eta}\N{iota}\N{omicron}\N{upsilon}\N{omega}]+
        \z
    }x; 
}