[kh99]

You can create a plugin using hook location register_addmember_process and code like this:

if (!empty($vbulletin->GPC['userfield']['field2']) AND strlen($vbulletin->GPC['userfield']['field2']) < 3)
{
   $userdata->error('phrasename');
}

Then you also need to create a phrase with the error message you want (set Phrase Type to "Error Message"), and replace 'phrasename' with the name of your phrase.