vendor determination in third party Sales Order

Dear Gurus,

Could you please let me know, how the vendor will be selected for the thirdparty sales? Is it possible to select vendor in the sales order stage or in any other stage?

I came to know in sales order processing for third party sales, when items entered in the sales order based on the material category, the sales order automatically creates purchase requisition. Now do we need to create PO for that PR or PO will also be created automatically..?

If i want to specifically select one vendor form the list of vendors..how to do?

Regards,

Sridhar.