Depending on your sort order, if the first and last record both have the
same vendor, then there is only one vendor. Probably a better option though
would be to run a query to open a recordset on the subform's recordset. The
only field you will need returned is the vendor's id field and you would
tell the query to return only unique ones. The number of records in the
recordset would be then number of vendors.

Example:
Dim rst As DAO.Recordset, intNumberOfVendors As Integer
Set rst = CurrentDb.OpenRecordset("Select Distinct [VendorID] From tblOrders
Where [OrderID]=" & Me.txtOrderID, dbOpenSnapshot)
rst.MoveLast
intNumberOfVendors = rst.RecordCount
Debug.Print "Number of Vendors = " & intNumberOfVendors
rst.Close
Set rst = Nothing