In case of Expense Entries, there seems to be no way to select a vendor and enter vendor expenses.
It is uncommon for vendors or suppliers to enter their own expenses. Although we recommend using Vendors Bills instead of entering vendor expenses elsewhere, it is possible to do so. In the Expense Entries screen, select Show: Vendor on the top-right to be able to log expenses for a vendor.
When you create a vendor bill, Core automatically creates corresponding time or expense entries for each item. On the other hand, you can create vendor bills by importing un-billed time and expense entries recorded by a vendor. When you mark expenses as reimbursable and approve them, Core automatically creates system vendor bills for them.