Hi,
I read the requirements and have just 2 questions:
1) would the over all tip amount be added to order total on paypal?
2) Do you have split payments like adaptive payment to transfer tip amount to relevant vendor concurrently or does the tip amount need to be recorded in their accounts?
With that being said, I can easily add this customization to your WC vendor plugin.
About me: I am an expert level wordpress developer with over a dozen custom plugins and several dozen custom themes developed in past several years.
Regards,
Asad