Sorry, obviously I didn't try any of this and to be honest I keep forgetting what works in templates. Try $count[count], it should be the same (you might also want to use some variable name other than 'count', it's kind of a common name). There's probably some "right" way to do this without creating a new variable.
I don't think you need to do anything like using ob_start/end. You may need to be using a global, however (I already forgot what the code looked like). I think this may be what template hooks are for, so you don't have to worry about variable name conflicts and globals. Maybe someone who knows more will enlighten us
ETA: Meanwhile, maybe try something like:
Code:
$vbulletin->userinfo['memberinfo_threadcount'] = $count['count'];
in the plugin, then use
Code:
$bbuserinfo[memberinfo_threadcount]
in the template.