Data Controls :: Populate A Gridview From Another Gridview Not Working

I have two gridviews on a form.  When the user selects a row from the first gridview the second gridview should populate based on the selected row.  The second gridview is not generating as expected.  If I remove the "@vendor_id_fk" from the WHERE clause of the query and replace it with a value, the second gridview generates when a row is selected in the first gridview.  So I know that's where the issue lies, but I am having some difficulty solving the problem.  

Here is the aspx: 

<div class="row">
<%--Vendor List--%>
<asp:GridView
ID="gv_vendor_list"
AutoGenerateColumns="false"
AllowPaging="true"
AllowSorting="true"
ShowFooter="true"

[Code] .....

I used the following as a guide:  [URL]..