Within memberaction_dropdown:
PHP Code:
<vb:if condition="$memberinfo['field7']">{vb:raw memberinfo.field7}<vb:else /><vb:if condition="$memberinfo['musername']">{vb:raw memberinfo.musername}<vb:else />{vb:raw memberinfo.username}</vb:if></vb:if>
I would think something like that, but you would really have to check it out for your self. I've not run this code, so I have no idea.