How can I use Liquid to keep track of payments made to Vendors outside of Liquid?

On the back of the Vendor card under Finance Details, you can Pay with Liquid if you intend to use Liquid to track Vendors and Work Orders but not to make payments.

Once you've made this change, you will be able to make internal notes on your method of payment. These notes are not visible to your Vendors.

In addition, you will want to manually mark invoices as paid directly in each Invoice. Do this by pressing  "Manually mark this invoice as paid outside of Liquid." on the specific invoice.