Step 1:

Product configuration:

- Product need to be "Can be purchased" and "Can be sold"

- On Inventory tab, enable MTO + Buy/Dropship route

- On Purchase tab, vendors need to be available on product vendor pricelist  

Step 2:

Create Sales Order:

- Choose route on Sales Order Line

- Choose vendor on Sales Order Line

- Confirm SO

Result:

A corresponding Purchase Order will be generated upon SO confirmation.