If your business is growing and this is an important part of the business, then you'll eventually regret not building in-house. Of course, you may regret poor execution on the in-house solution, but the response to that is to start small and incrementally grow scope instead of trying to switch over a big mission-critical system. All big functional systems start as small functional systems and grow incrementally. It's easy to imagine the system you'd like to end up with, it's hard to imagine a series of small changes to get there from where you are. But if you want it to be successful, you have to take the longer path that is in production the whole way through.
Consider having the new development focus on features you don't have in the current vendor solution instead of replacing features you already have. These will be more impactful and give the new team some positive inertia.