I recommend using vendor attributes for this purpose, e.g. add custom attributes for the payment method credentials that you allow for payouts in Vendors/Attributes, mark them as Editable and Required. This way vendors will have to fill these fields, and they will not have to fill them every time on requesting a payout. As a site admin, you’ll be able to always check them in the vendor profile in WordPress/Vendors section.
Adding custom fields to the payout form, validating, saving and displaying the field values somewhere would require advanced customizations.