If you send multiple messages to a single send port, the messages are only lined up and sent one at a time it you set Ordered Delivery. Otherwise you can have multiple instances.
In your scenario, you can only use a dynamic port if you perform your maps inside the orchestration, so you will need a large decide shape and knowledge of each vendor inside the orchestration. This will need to change everytime a new vendor was added.
By having multiple send ports, each send port can have its own map (and even protocol) while the orchestration does not need any vendor specific code.
By using a direct orchestartion port you will need to promote the property used for routing. If this property is set inside the orchestration it is only promoted if you create a correlation using this property type and initialise the correlation on the send port.
As RussW mention previously, this scenario is what RoleLinks are designed for.