Replace those variables with null and you will be left with , (comma) after the name.
Code:
<vb:if condition="!$show['detailedtime']">
{vb:rawphrase started_by_x_y_z_a, {vb:link member, {vb:raw thread}, null, 'postuserid', 'postusername'}, {vb:raw thread.postusername}, null, null, {vb:stylevar dirmark}}
<vb:else />
{vb:rawphrase started_by_x_y_z, {vb:link member, {vb:raw thread}, null, 'postuserid', 'postusername'}, {vb:raw thread.postusername},null, {vb:stylevar dirmark}}
</vb:if>