you can try below code instead of for loop - because you have only 2 items to findout.
string serial = TextBox8.Text;
string[] s= serial.Split(',');
dim a as string = s[0].tostring
dim b as string = s[1].tostring
string qry = "SELECT DISTINCT Vendor_Name FROM [Vendor] WHERE Asset_Type in( '" + a +"','" + b +"')";
SqlDataAdapter adpt = new SqlDataAdapter(qry, con);