This refers to the actual online races and not the asynchronous multiplayer aspect of V Rally mode. Online is completely dead. Grab a boosting partner or a 2nd Xbox to get this (and the other MP achievements) done.
Starting a race is not enough - you must finish one race of each event to unlock this:
Hillclimb (descents also count)
Only 2 players are required to start an online race. There is a 30 second countdown timer once the 1st human driver finishes. To get credit for the race, you must finish before the timer expires.