I agree that it does not appear to be a valid SSHA256 hash but it would help if we knew the salt. Here's what I came up with after some searching.

#!/bin/env perl
use strict;
use warnings;

use MIME::Base64;
use Digest::SHA qw( sha256 );

# Only the first one can be validated...
my @passwords = (
    {
        encoded     => 'M8Nbe/nfLJeVbV3XdvaLD44uxK77eAhJWEx1tIDLBCFteV
+9zYWx0Xw==',
        password    => 'P@ssw0rd!',
        salt        => 'my_salt_',
    },
    {
        encoded     => 'B6HO7UNHVi5fglh1RpJXX4z1maGJ9lcicTVcy94ztsmzAe
+kseg==',
        password    => 'Passw0rd!',
    },
    {
        encoded     => 'KGOnPYya2qwhF9w4xK157EZZ/RqIxParohltZWU7h2T/VG
+jNRA==',
        password    => 'VMware1234!',
    }
);

for my $pass ( @passwords )
{
    print "encoded: $pass->{encoded}\n";
    my $decoded = MIME::Base64::decode_base64( $pass->{encoded} );
    print "decoded: $decoded\n";

    # Find digest and salt
    my @salt = unpack( 'C*', $decoded );
    my @pass;
    push @pass, shift @salt for ( 1 .. 32 ); # shift the password off 
+leaving just the salt behind
    my $pw = pack( 'C32', @pass );
    my $salt = pack( 'C*', @salt );

    print "password digest: $pw\n";
    print "salt: $salt\n";
    print "     salt matches what is stored\n" if ( defined $pass->{sa
+lt} && $salt eq $pass->{salt} );

    # Create a new password digest
    my $digest = sha256( $pass->{password} . $salt );
    print "digest: $digest\n";

    my $reencoded = MIME::Base64::encode_base64( $digest . $salt );
    chomp $reencoded;
    print "reencoded (SSHA256): $reencoded\n";
    print "     reencoded matches the original encoded value!\n" if ( 
+$reencoded eq $pass->{encoded} );
    print "\n\n";
}
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