[kh99]

OK, in member.php there's this code:

if ($vbulletin->GPC['vmid'] AND !$vbulletin->GPC['tab'])
{
	$vbulletin->GPC['tab'] = 'visitor_messaging';
}

So maybe if you set $vbulletin->GPC['tab'] to your tab at hook member_execute_start. Like:

if (!$vbulletin->GPC['tab'])
{
	$vbulletin->GPC['tab'] = 'my_tab';
}

Edit: ...or maybe you also need to check $vbulletin->GPC['vmid'] like the code above. I don't know what that is.