Since it is private is there a way to replace solidity with vanilla JS?
You'd need to write a compiler that could compile JS to EVM bytecode, and I'm pretty sure no one has done that yet.
The accepted answer to the following question might help explain the rationale behind developing a new smart contract languages, rather than use an existing language like JS -> What is the merit of creating new smart contract languages like Solidity instead of using other languages?
The gist is along these lines:
"Every programming language is designed for a particular operational environment and target tasks; and these constraints drive almost all design decisions on what features to support and which ones to drop."
You don't have to stick to Solidity. Here's a list of languages that can be compiled to EVM bytecode: https://github.com/ethereum/wiki/wiki/Ethereum-Virtual-Machine-(EVM)-Awesome-List#programming-languages-that-compile-into-evm