[kh99]

I think you need to add this line (before or after the other register() call):

$templater->register('timeframe', $timeframe);

and then in the template:

<vb:if condition="$vboptions['posts_last_x_hours_enabled'] == 1">
{vb:rawphrase posts_in_last_x_hours, {vb:raw posts_last_x_hours}, {vb:raw timeframe} }
</vb:if>

BTW, there's some info in the manual that might be helpful: https://www.vbulletin.com/docs/html/ (search for "template syntax")