Nice, lightweight, useful.
There is a missing bit of instruction in the OP however.
In your code you call to "$vbphrase[memberinfo_visitormessage_reply]" which does not exist in the phrases.
Instead of replacing it with the text "Reply" as previously suggested, I simply created a new, global phrase with the Variable name:
memberinfo_visitormessage_reply
and the text:
Reply.
Voila.
*NOTE* Another good thing about this MiniMod. It's not AJAX, so there should be no mobile skin issues.