User wants to give the vendors access to Core so that they can directly enter time (services). Do they need to purchase a subscription for the vendors? Does Core allow them to create vendor bills from those time entries?
You can allow your vendors to use Core by creating vendor records of type Contract Employee for them. You also need to purchase a subscription for the vendors and assign it to them. Check out this article for more: How can our vendors access Core.
You can easily create vendor bills from the vendors' time and expense entries. Check the Core Help for details on that.