I'm in the process of similar transition in global manufacturing company, where my role is to build internal engineering team to essentially increase ownership and remove reliance on external vendors. So far I would say it worked, but with a few caveats. One key thing to note is that usually you don't have to build e2e team to replace your current vendor, as replacing key roles might be good enough to drive visible positive change.
What I would suggest is to divide the work scope into smaller chunks, eg. isolate applications / systems. This will allow you to distribute the work to other vendors or your own in house IT team. The change usually requires a lot of work in different areas, such as clearly defining people functions, R&Rs or work organization. Then you insource key roles, and see if the effects are what you're expecting. Having some local CTO type of person to drive the change would definitively help.
Overall I would expect the process to take several quarters, highly depending on your system complexity and vendor lock-in (they will not be happy about the change!).
I've seen in the other comments reco to find freelancers instead of FTE, and it could work depending on local market specifics. In some countries you have to watch out for employment laws and potential issues. I'm based in Poland so can provide more info if you want to chat (myr11242@gmail.com). Good luck regardless :)
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