Clue by Clue: The Betrayed Trader Solution
The Vice President poisoned Wentworth’s coffee. We know Wentworth locked himself in his office in the morning, so the only time anyone had access to his coffee was before or right after the courier came. Wentworth was making coffee and we know he left his mug in the kitchen when he got the reports. The courier saw the carafe, but no mug.
So who was at the office early? Most likely the Vice President. Remember she was talking on the phone about the reports, so she must have seen them. She likely came to work early to get rid of Wentworth before he could punish her for her mistakes.