How do I set a a databound dropdown to a dataset value

How do I set a a databound dropdown to a dataset value

I have a drop down that is bound to a set of values. When the user selects
a value from another drop down the form gets populated. If the value
exists in the database I want the value to be already chosen in the drop
down. But with the option to choose a new value.
Note the dataset is bound at this point. I have tried
DataSet dsGetOrders = new DataSet();
foreach (DataRow dsGetOrdersRow in dsGetOrders.Tables[0].Rows)
{
ddpDown.SelectedItem.Text = dsGetOrdersRow["Vendor_Name"].ToString();
}
But this just adds the row in the drop down and doesn't make it the select
value.
The drop down DataTextField and DataVaule are the same field. I even tried
this without success:
ddpDown.DataTexField = dsGetOrdersRow["Vendor_Name"].ToString();