It's remaining selected because you made it selected all the time by removing the condition around it.
Yours:
HTML Code:
<li class="selected"><a class="navtab" href="{vb:link forumhome}">{vb:rawphrase forum}</a> <ul class="floatcontainer">
Default:
HTML Code:
<vb:if condition="!$vboptions['selectednavtab'] AND THIS_SCRIPT != 'search'">
<li class="selected"><a class="navtab" href="{vb:link forumhome}">{vb:rawphrase forum}</a>
<ul class="floatcontainer">
You need to compare yours to the default.