ASP.NET

public class SomeStuff
{
    public static void DoSomethingWithTheVendor()
    {
      ... get the currently selected vendor from the drop down list...
      ... do something with it...
    }
}

public class SomeStuff
{
    public static void DoSomethingWithTheVendor(string vendorId)
    {
      ... do something with the vendor, given the passed-in vendorId...
    }
}

void MyButton_ClickEventHandler(object o, EventArgs e)
{
    // get the currently selected vendor
    string vendorId = myVendorDropDownList.SelectedValue();

    // call an App_Code function to operate on the selected vendor
    SomeStuff.DoSomethingWithTheVendor(vendorId);
}