iTunes Match wont match a song in the store that i already own, recorded from vinyl
Surely the waveform of my vinyl recording is the same, regardless of gain level?


Unfortunately it does not appear to be that simple. Many factors can affect the quality of a digitised vinyl recording such that it will not match the digital fingerprint Gracenote is comparing against.

There is an interesting discussion on Apple Support Communities with suggestions on how to improve the digitised vinyl version so that it better matches such comparison services where possible:

https://discussions.apple.com/thread/3507807