Tedious Reverse engineering project.
IMVU staff has a github account where they posted the core code that handled requests on their server ... well over a decade ago. The code is highly outdated and incomplete, but gives you a good idea on how complex IMVU is on the server side. There are also IMVU engineering blogs that map out the network and its complexities. Any attempt at making an IMVU private server take about a team of 2 - 3 programmers about 2 years if my estimates are accurate, which normally they are. That team would need to contain programmers who are already well versed in IMVU's systems, as well as extensive knowledge of networking and server administration if you plan on the server actually handling a decent sized population. Chat rooms can be one of the most resource intensive systems to host on a server ... so many repeating connections and heat beats.
You would be better off investing time in making an IMVU unity mod. You still get to make your own IMVU functionality that way, and you can legally make money off of it. Not to mention you would actually get to spend time programming and developing instead of reverse engineering someone else's system.