We are still going through this where I work. Previously IT was run on a bunch of Lotus Notes / Domino databases. Those have since been replaced by PeopleSoft and ServiceNow.
You have to see the opportunity for what it is. You can have real conversations with the departments about what their real needs are. It is going to take a while, but you will have to produce documents that detail the core application functionalities for all of the applications. Then you will have to map those functions into the ERP system. Once you have done that, you will have your gap analysis and be able to focus your developmental resources. You have to get buy in from across the organization and get people committed to and willing to do things differently. The ERP equivalents of the current applications will not be apples to apples. If you try to do that, you will never get through it and will end up failing. If you are just going to recreate the apps, you might as well not even bother. The key is to focus on the functionality. Focus on the business needs / business cases for the applications.
For something that big, you are going to need at least 3+ full time employees. A project manager to keep everything organized and fight back against scope creep, a senior developer / architect to make the technical decisions and provide guidance to the team of developer(s) who will do the actual work. In all honesty, what you are proposing is a significant investment for the organization and a shift in culture. Each one of those employees is easily a six figure salary, so figure over half a million dollar in salary (plus benefits, etc.) Good developers are hard to find and building a successful development team is a challenge. You will obviously need an executive sponsor who can help you figure out where to position this new group / department in the overall organizational hierarchy.
The long term benefit to your organization is that you free yourself from the vendor. The risk that you run is that you might end up with incompetent developers or management on the new team and find yourself worse off than before.
Have you considered bringing in another vendor? At the very least, you can use that as leverage to negotiate more favorable conditions with the current vendor.
You should have enough experience with the current vendor to determine how accurate their project quotes are. Use that knowledge when you ask them for quotes on replacing / reproducing the current application functionality. Then compare that to what it will cost your organization to do it in house. It should be clear very early on in the process if you are going to save enough money to justify such a drastic realignment of the management, operation and development of the systems.