How do I start a payout for everything a vendor has earned so far?

Start the payout from the Payouts page.

1. Go to Vendors → Payouts.
2. Choose the seller in Vendor.
3. Click Initiate payout.

This bundles all of that vendor's unpaid earnings into one payout with the status Processing. Those earnings can't be included in another payout.

Wcart doesn't send the money itself. Pay the vendor by bank transfer or UPI, then click Mark paid on the payout and enter your transaction reference. If you see "No unpaid earnings for this vendor", first run Sync from orders on Vendors → Vendor Earnings.