Are you familiar with HTTP requests..? I took a quick look, and provided you managed to get ahold of an authenticated session, all it appears it would really require would be to write something that POSTs to /vb/newreply.php... A simple feat in most languages.
For example, the request for this post initially looked like this:
Code:
Name Value
securitytoken <snipped>
ajax 1
ajax_lastpost 1370724159
message Are you familiar with HTTP requests... Provided you managed to get ahold of an authenticated session, all it would really require would be to write something that POSTs to ajax.php...
wysiwyg 0
styleid 0
fromquickreply 1
s
securitytoken <snipped>
do postreply
t 38888
p who cares
specifiedpost 0
parseurl 1
loggedinuser 6599
The securitytoken I believe is the PHP session ID, whereas the remainder of the parameters are pretty self-explanitory.