I would pass up an XML such as:
<clauses>
<c clientid="287">
<vpi product="SX" />
<vpi product="SY" />
<vpi product="SZ" />
</c>
<c clientid="559">
<vpi product="D7" />
</c>
<c clientid="288">
<vpi product="SX" />
<vpi product="SI" />
</c>
</clauses>
You can then select against this structure to product a table representing these values:
He're an example on selecting a data set from XML
Select values from XML field in SQL Server 2008[
^]
Client Product
287 SX
287 SY
287 SZ
559 D7
288 SX
287 SI
You then
INNER JOIN
this to your product database to filter out the desired products.