Programatically insert/update lookup field with data from external list

Hi

I have a SharePoint list called Products which has many Look Up column. I also have multiple external lists which I use for the look up fields in Products list. I am at a stage where I have to import 1000+ items into Products list programmatically. Here is what i have so far.

var client = new Microsoft.SharePoint.Client.ClientContext("http://intranet.ltp.org/products");
var list = client.Web.Lists.GetByTitle("products");
var newItem = new ListItemCreationInformation();
var newListItem = list.AddItem(newItem);

newListItem["Item_x0020_Code"] = "3DR"; //Text field
newListItem["Vendor"] = ??? //Vendor is a lookup field that uses an external list. I tried setting the field with a text value, but it goes away when I edit the record after being recorded. I tried setting the BdcIdentity of the actual value, but that goes away when i edit the record as well.

newListItem.Update();
client.ExecuteQuery();

Can someone point me to the right direction? Or is there a work around to get look up field value that uses external list data programatically?

Any help would be really appreciated.

Thank you,

Amal