auto secondary usergroup

[White_Snake]

0k, i have codded this small plugin that is supposed to assign a secondary usergroup when the user reaches 50 posts, all based on a question asked on the register form, "Choose Letter A or B" so, this is supposed to work on that way, if a registered user have choosen A when they register, when they have reached 50 post, they should be able to be on the secondary usergroup "A" or of they have choosen B, the same effect has to occur when they reach 50 posts to be on the secondary usergroup B

Hook: newpost_process

PHP Code:

if($vbulletin->options['field5']=="A" AND $vbulletin->userinfo['posts']=="50")
{
$vbulletin->userinfo['membergroupsids'] = 9;
}

if($vbulletin->options['field5']=="B" AND $vbulletin->userinfo['posts']=="50")
{
$vbulletin->userinfo['membergroupsids'] = 10;
} 


but that event isnt happening, so im not sure where is/are my error(s)

maybe the hook, maybe some wrong codding or maybe both things

thanks for your time