So, I finished, and then revised, the front end of the "Secret Negotiations" and have mostly worked out the factors for success. Here is what the revised version looks like, with the same target as before:

- Remote Negotiations.jpg (183.64 KiB) Viewed 67 times
Now that is the chances for a "remote" negotiation. The other options, "Estates & Pension" and "Senatorial Status" give better chances but also cost more money. The "Estates" option requires 1,500 pounds of gold, and the "Senatorial Status" offer will cost the player 1,000 pounds of gold.
Just to illustrate, here is what the Tool Area looks like with Alexandria selected:

- Indirect Offer.jpg (61.79 KiB) Viewed 67 times
However, there is another option for the player. If Vespasian moves by sea to Alexandria, he can use the same Ops Command to make, in effect, an offer in person:

- Direct Offer.jpg (67.21 KiB) Viewed 67 times
As you can see, there is a bonus for doing this face-to-face. Vespasian can do this because he isn't at war with Otho at the beginning of the scenario. His forces are allied with Otho and the Senate. (Once he is acclaimed Emperor it's going to be a different story, of course).
With that bonus, here are the chances:

- Chances In Person.jpg (186.56 KiB) Viewed 67 times
The completion of the command just uses an animation with a message in the tool area:

- Tool Area Message.jpg (77.66 KiB) Viewed 67 times
Admittedly, it's rather obvious. I will add a notification event and post a screenshot of that next week.
The "Vox Populi" line indicated a possible adjustment in the offering leader's Auctoritas and Opprobrium values. Granting Senatorial Status can create friction with the Roman Senate if the target leader is obnoxious. That is measured by the target leader's current Opprobrium value. In this case, the effect on such opinion is negligible, hence the value "Indifferent."
"Vox Populi" literally means the Voice of the People, of course, but here it is the opinion of "upper crusty" kind of people that is being estimated.
P.S. Leaders can move by sea without a fleet or a transport because it's assumed that they can commandeer or rent a single ship. Of course, if there are hostile fleets at sea then it won't be safe to do this.