Is there a way to set up a compression VST on the default input of my mac? e.g. I want to make a skype call and I have my USB soundcard as mic input, is there a way to set up the default input as my soundcard passing through a compressor (for high gain purposes)?
Your question is very unclear, however, unless your undefined 'USB soundcard' has an AU-compatible front-end or its own input gain stage, then you'd have to use something like Audio Hijack.
You can't easily use VSTs on the input without an ASIO/VST controller & even then, they would normally be strapped to the output... which you would then need something like Audio Hijack to re-route...
[Previously, SoundFlower (freeware) would be the go-to solution, but since Mavericks it is no longer stable]