Filesystem or whole-drive encryption would be the simplest solutions.

If you must encrypt VSAM files separately, you could do it with a custom file handler - see your product documentation for details. But you would have to find a way to deal with key security and hygiene (a hard problem), you'd have to trust your cryptography implementation (a job for experts), and performance would almost certainly be worse than with filesystem or drive encryption (which can take advantage of filesystem/drive structure and use fast modes designed for this purpose such as XTS or PFB.