[calorie]

Untested but look in the vB SHOWTHREAD template for <!-- thread tags --> and in the template conditional preceding that line, add in the part in bold red:

<if condition="$show['tag_box'] AND in_array($forumid, array(1,2,3))">

Where 1,2,3 are the forum IDs where you want the tags box to show on the showthread.php page.

Alternatively and also untested, but if you want to completely disable tags in certain forums, a plugin like the following at the global_start hook should work:

// 4,5,6 are non-tag forum IDs
if ($forumid && !in_array($forumid, array(4,5,6)))
{
	$vbulletin->options['threadtagging'] = 0;
}

Adding that plugin will also remove the tag search from the navbar when in certain forums, so if you don't want that to happen, remove the threadtagging template conditional from the vB navbar template, assuming thread tagging is globally allowed in the vB options.