It's a liitle bit easier to just write :

<a href="#" onclick="vbscript:OpenWindow3('{<xsl:value-of select="@SSN"/>}',{<xsl:value-of select="@Plancode"/><xsl:value-of select="@Plancode"/>})"> <xsl:value-of select="@SSN"/></a>

I suppose you get the error message because of 999-99-9999 is a string, and not a number, so just put single ' around the xsl:value of the first argument in your function