You should collect a signed Form W-9 at the time you have the vendor sign the subcontractor agreement. It's always a best practice to have the form included as part of the onboarding package for when you hire a vendor or bring a subcontractor on board with your team. At the latest, the Form W-9 should be provided before you remit payment to the vendor or contractor. If you don't receive the signed Form W-9, you are technically required to impose backup withholding upon the gross payment and remit the taxes to the IRS.