> and SEARCH_STRING is defined as
> <xsl:param
> name="SEARCH_STRING"> and ( (related/@code = '111') or
> (related/@code = '222') or (related/@code = '333') )</xsl:param>


assuming you are using xlt1, that defined $EARCH_STRING to be a result
tree fragment corresponding to a root node and a text node child with
the text 
"and ( (related/@code = '111') or
(related/@code = '222') or (related/@code = '333') )"

result-tree fragments, if used in a boolean context as you have used
them always evaluate to true as they correxpond to a node set which
contains a single node (/) so are not empty.

variables in xpath (as in most other programming languages) store
values, not expression fragments.

If you are using using xslt2, you could use

<xsl:variable name="myNodeSet" select="site/assets/asset[(type/@code =
'aaa') and starts-with(@vpath,'my/path/')
  and (lang='us_en') and related/@code = $sequence"/>



<xsl:variable name="sequence" select="(111,222,333)"/>

In xslt1 you could do something similar but probably most conviently if
you have access to the node-set extension.

David

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