Hi,

it would be quite hard, since when purchase orders are created you do not have info about a sales order.

Generally, what you should do:

1. Add in sale.order.line info about a supplier as m2o to supplier info (the same as in a product)
   

2. Somehow transfer SO this info through procurement. E.g. have a look at the method _prepare_order_line_procurement of sale.order and at how a chain of procurements is generated in the module 'procurement'. 

3. Then in the purchase.order method _check_supplier_info add a check from this info, instead of getting it from a product.

Simplier way would be to define some logical parameter how to select a supplier. So, you add some extra parameter to supplier.info (such as availability for example) and use this parameter to _check_supplier_info to select a proper vendor.

Hope that helps.

I can't edit the topic so I will explain again my issue:

When I create a sale.order, when adding a product in the sale.order.lines the supplier is selected automatically depending on some criteria, that is ok, the common behaviour, but if two or more suppliers have the same criteria, always the first one is selected (and the purchase.order created automatically from MRP module is always for the first supplier), my issue is that i need to be able to select the supplier within the same criteria when creating the sale.order because sometimes the supplier selection is related with external apart away from odoo (supplier availability or other business logic not related with odoo)