How to Maintain Separation in Small Business Environment?

I work in a small business, where IT is currently outsourced. They have VNC access to a cluster of our computers, whereas a separate cluster of computers is completely offline.

Is there a secure way of setting up my network such that I can bring the second cluster online while preventing the IT company access to our proprietary files? Would a managed or Smart switch help resolve this problem? What would a VPN allow me to do?

This may be a novice question, but any help or further resource is appreciated.