How will I know when the vendor has made their marketing deposit payment?

You'll have full visibility every step of the way.

From within the workspace associated with the transaction, you can keep track of where things stand at any point. Once the deposit has been requested, the status will show as "Requested". When the vendor makes their payment, or lets you know a bank transfer is on its way, this updates to "Payment Initiated". Once the funds have landed, you'll see the status move to "Paid".