User wants the vendors to be able to see what they are owed in Core. The vendors are added as Core users with the Time & Expense subscriptions.
If you have vendors with the Time & Expense subscriptions, they can see the rates that they are getting paid at by exposing the Cost Rate column in the Time Entries and Expense Entries screens (More > Show/Hide Columns > Cost Rate). Check the Core Help for more details. Additionally, they can see the cost rates by running the Time Details or Expense Details reports. Check the Core Help for more on running reports.