I found that it's possible for mother to not recognize that you're done with the last arc and not give the briefing. You just miss a quick explaination though. They tracked him to Vesuvius and the whole area is a free fire zone.
No trick to this one. Just take in the scenery and see if your computer can handle all of the falling ash and crowds without lag. Just head along the linear path until you get to the gates. Walk inside and down the street. Turn into the first courtyard to meet up with Dexter and our new friend Bob. Bob's pretty capable, so don't take your orders as an escort mission. Go out the little side door to start the battle with the opposition.
The problem is that you're basically in a random battle. The opposition spawn in pairs at their portals and your agents spawn at any captured portal. After that, it's all up to the AI and what they can do. For now, just focus on shooting your way across to the closest portal. Kill the few guards that should be in place, and focus on switching the calibration. You don't have to kill all of the guards first, just get it switched so that reinforcements start coming through. I also suggest that you trade out the AR 55 for an M7 rifle from a downed opposition agent.
The second portal is also right on the path and easy to find. It's just around the corner after the broken part of the street. Take them out and reverse it. Then charge forward and toward the next portal. You should be able to see it on the other side of a fence. Go down the street to the right until you get pulled into a little cutscene that introduces the automatic turret. We'll deal with that in a minute. Just get off the street and behind the fence with Bob and the rest of your men. Charge inside and gun down the agents here. The pillars make for great cover.
You need to blow up the turret now. Step outside and crouch behind the barrels. Then quickly move up to the crashed cart. Heal up for a minute, and then sneak over to the left and get behind the fence. This should be out of it's range. Move forward and quickly cut back and up the slope. Plant the charge on the side and blow it up. 
Once that's done, you need to charge up the street and knock out the next portal in the side room. Nothing new here, since you just need to gun down a few more agents and switch it. Then push forward and grab the other one up and to the right. The only interesting note is that there may be some Roman guards left, who have the absurd ability to deflect bullets with their shields. Just line up a head shot or shoot them when they move their shield away.
You need to move a bit further down to the marketplace. Before you bother engaging the guard at the amphitheater, go down and to the right to knock out the portal in the little side room. I'm having you start this one up because we'll need the reinforcements to take the full marketplace. On that note, there are probably a ton of agents moving in on your position. Take cover inside and ambush them as they rush the door. The shotgun and the M7 are fine. Then leapfrog from stand to stand until you can get to the portal outside and reverse it. Once it's flipped, backtrack and go down the side street to flip the new portal that should be on your map. You'll have to go around a fruit cart, but it's not much of a challenge. Just duck inside and then back out.
Once that's done, you should have an army in the marketplace holding back the opposition. Just give one last push to reverse the last portal in the back. Once the amphitheater entrance is clear, plant some C4 on the door and blast your way in. Then go inside to find the opposition's main base.