Tags have already been incorporated in the GARS_newthread template. To manually add tags, find $posticons in the GARS_newthread template and beforehand add the following:
<if condition="$show['tag_option']">
<fieldset class="fieldset" style="margin-top: $stylevar[cellpadding]px">
<legend>$vbphrase[tags]</legend>
<div style="padding:$stylevar[formspacer]px">
<div>$vbphrase[separate_tags_using_comma]</div>
<span id="tag_add"><input type="text" class="bginput" name="taglist" id="tag_add_input" value="$newpost[taglist]" size="40" /></span>
<if condition="$show['tags_remain']"><div><phrase 1="$tags_remain">$vbphrase[you_may_add_x_tags_to_thread]</phrase></div></if>
</div>
<if condition="$show['popups']">
<div id="tag_add_menu" class="vbmenu_popup" style="display:none"></div>
<script type="text/javascript" src="clientscript/vbulletin_ajax_tagsugg.js?v=$vboptions[simpleversion]"></script>
<script type="text/javascript">
<!--
vbmenu_register('tag_add', true);
tag_add_comp = new vB_AJAX_TagSuggest('tag_add_comp', 'tag_add_input', 'tag_add');
tag_add_comp.allow_multiple = true;
tag_add_comp.set_delimiters('$tag_delimiters');
//-->
</script>
</if>
</fieldset>
</if>
Not sure what to suggest for vBCMS, as I don't use vBCMS and am not familiar with that code.