I agree, that another firm may be the best option in the short term. Hopefully, you had a change management clause or agreement set up in your contract, that keeps your current firm from completely ditching you without a proper hand-off. Because it will take you some time to hire a your own dev (This guy will probably be your CTO & co-Founder), you'll need your new firm to comprehend, apprehend and decipher everything your old says. One of you will need to participate in this dialogue and you have to require that your new firms does great documentation (to the point that all three of you understand).

This will not be inexpensive and finding an individual to hire will require more than a stake in your company in a small salary. You may want to consider a reasonable or slightly above stake and an average salary. You're asking a lot of any individual to come into a situation and repair it. At this point, you need this person more than they need you. There are a lot of executive decisions to be made but it goes with the territory of a startup.

You face two huge challenges outside of the dispute with your outsource firm. Hiring a capable individual that can do all that you are requesting now and in the future. Convincing a VC firm that this is not a huge problem.

Good Luck