What do you mean by making it possible to choose multiple vendors ?! i certainly would not advise having a O2M instead of the M2O in the PO. 

I think what you're looking for is "Call for tenders" : A purchase agreement where you send your order to multiple vendors. Once they contact you to give you their prices, you can select the one that suits you. ( check : https://www.odoo.com/fr_FR/slides/slide/call-for-tenders-626?fullscreen=1 ).

Hope this helps.
you can write back for further analysis.

Regards