As was mentioned in another answer, vehicle customization pieces in Mario Kart 8 unlock by collecting coins; with intervals of 50-100 set as criteria.
However, it seems that while the manner of unlocking items is stable, what you receive is not. For example: The unlock guides provided by IGN and Gamers Heroes both claim that the Yoshi Bike is awarded at
600 coins. However, although I have already collected more than
600 coins (from the unlock messages, I estimate I'm approaching the
1400 mark), I still have yet to receive the bike in question. Other players around the internet seem to have similar situations to mine; for example: this comment by Sharon.
I have also heard mention that perhaps the vehicle customizations' unlock order is random, the class of item you recieve isn't. For example, at the
600 coin mark some sort of bike would be unlocked, not necessarily a specific one. I can't find any information directly in favor or against this rumor.
So in short, is there any rhyme or reason to the order in which vehicle customization pieces are unlocked? and if so, what the heck is it! :)