Hi,
When the customer is redirected to Paypal, the currency sent to paypal is the currency of the order.
And to be more precise, it is the currency that the customer selected in the currency switcher.
If you do not have a currency switcher, the currency would be the store default (main) currency.
At this moment the best will be to have a plugin which will change the currency of the order (the used currency for the customer) depending on the products in his cart.
So when the customer will add a product in his cart, it will set the used currency as the vendor currency.
Thanks to the vendor limitation in the cart, you won't have bad side effect.
I don't see nice way to do so in the paypal adaptive plugin because it must use the currency of the order, otherwise the IPN validation or some other stuff (like the processing of the fees) could be affected (become wrong).
Regards,