You are not Linking the Vendor Name: "If the later, how do I 'link" them both back to John Jones (vendor)?"
These are Customer Activities. When you pay for 5 signs for 5 different agents, you enter the split details as Billable to each agent using their Customer Name on these details. That is Billable to each of them.
You can use Statement Charges with the Add Time & Costs function, or Invoicing. Statement Charges bypass the use of an invoice, so no ability to include "items that calculate" such as Sales Taxes, markups, discounts.