[Scanu]

Even the variables you see in vbulletin templates aren't global you have to register them again if you want to use them in another template
There are only a few variables and costants that are availavle in every template
However the last visit and activity code is very easy you could make a new plugin

Plugin hook: forumhome_start (should work)

PHP Code:

$lastvisitdate = vbdate($vbulletin->options['dateformat'], $vbulletin->userinfo['lastvisit'], 1);
$lastvisittime = vbdate($vbulletin->options['timeformat'], $vbulletin->userinfo['lastvisit']);
vB_Template::preRegister('FORUMHOME',array('lastvisittime' => $lastvisittime, 'lastvisitdate' => $lastvisitdate)); 


And then you'll be able to use {vb:raw lastvisittime}, {vb:raw lastvisitdate}
I didn't test it so I'm not sure it will work but you can play this plugin and see if you can get it working by yourself, if you can't feel free to ask further help