[kh99]

I'm a little confused because you say that it currently looks for the key 'name' and I don't see that in the register.php code anywhere (I also don't see any do='step2'). But anyway, maybe something like this (using a plugin on hook register start):

$vbulletin->input->clean_gpc('r', 'username', TYPE_NOHTML);

if (empty($vbulletin->GPC['username']))
{
    $vbulletin->input->clean_gpc('r', 'custom user', TYPE_NOHTML);
    $vbulletin->GPC['username'] = $vbulletin->GPC['custom user'];
}

That way it'll use 'custom user' if it exists and there's no username already specified (if you want 'custom user' to always override the username, then remove the 'if' statement but leave both clean_cpc() calls).