Europa Universalis Rome: Vae Victis is the expansion pack to EU:Rome. Unlike other Paradox Expansion Packs it doesn't increase the timeline but instead is solely focused on gameplay mechanics. It's aims are quite similar to that of Hearts of Iron 2: Armaggedon.
Characters can now recieve missions. Another addition is the Senate which comes with any republic and gives the player missions to complete. There are also 5 political parties. Military, Mercantile, Religion, Civic and Populist. Whichever party has more control determines what diplomatic options you can unleash. Characters can now also be given titles which can further boost there stats or weaken those of your rivals and each character is also part of a political party of there choosing. Generals have also been modified for Republics and only characters that havebeen in a position of power can be given that position or the Senate themselves will appoint someone to it.
There are 3 main government types, Republic, Monarchy and Tribe. A Republic has the 5 different parties; Military, Civic, Mercantile, Populist and Religious who all fight for seats in the senate. Whichever Party is in control determines different bonuses and different diplomatic options, eg The Military Party is likely to have more in favour of a declaration of war. The Populist Party however gains seats from dissenters and doesn't benefit your nation if they get in power as they tend to switch around national ideas and reduce modifiers.
A Monarchy follows a single family of rulers who rule the nation by decree, this means there are no votes on whether or not you can use diplomatic options. There are also several positions within the realm such as treasury which can be assigned to other characters and the better their stats the more of a benefit that position gives. Typically the eldest son gains the throne although laws can be passed which can alter the order slightly.
A Tribe has several different clans within it who all normally demand positions of power within or else stability tends to suffer. It is a mix of Republic/Monarchy in that there are 5 familes fighting for power and alterng power but one overall ruler.
The mission/decisions system from In Nomine has been implemented. The Senate gives the player missions and completing them gives you rewards or failing to complete them can have drastic consequences. Laws, have replaced the traditional slider system that was used throughout other Paradox Games, They can be passed and they can also be taken away. However stability hits are taken by repeatedly passing and taking them away. Decisions can also be used to change the government type and also offer other benefits such as extending the road network of Rome which was a Historical Decision.
Provinces have also been altered. There is no longer a governor for each province but now for each region which is typically made up for 5 provinces which has greatly reduced micromanagement. Technology has been tinkered with slightly but remains for the most part unchanged and is now part of the Monarchy screen as well. The Interface overall has undergone a overhaul with the ambition to make more infomation availble to the player and to make other screens such as diplomacy easier to get to. The AI has also had a big improvement with it's military in order to give more experienced players a challange who quite frequently were able to conquer the majority of the known world.
Currently version 2.1, Another Patch is in the planning but the team is focusing on Hearts of Iron 3.