Place 1st in every career race.
At anytime in career you can select and retry any event you wish. Most events throughout career are easily won with a regular vehicle, but always choose wisely because the locked vehicles are not always the best out of them all. You need to gain the highest points possible in an event, but that shouldn't be an issue. The only events that will pose an issue will be the "beat the lap time" races, and for those I suggest racing them once and getting what you can. Don't try to beat these events until you have at least unlocked the final Dirt Bikes, only then and while driving one should you attempt to beat the lap times. I found the Blue Lightning Dirt Bike to be the fastest vehicle in the game.