Historically, there was an API translation layer that VST2 plugins used (called Symbiosis), but these days the vast majority of plugin devs use a framework like JUCE which has multiple different "client-side" API implementations (of VST2, and VST3, and etc) that all wrap around JUCE's native class hierarchy.

There's a few other frameworks floating around (Dplug for writing in D, a few others in C++), but JUCE is far and away the most common.