Edacade's right,
Doubling Season did not work on a vivid land played from your hand before this rules change. The reasons are obscure and weird, which is why this is a very good change.
Basically,
Doubling Season specifically mentions "effects" and the rules have a specific definition of what counts as an effect.
Quote:
609.1. An effect is something that happens in the game as a result of a spell or ability. When a spell, activated ability, or triggered ability resolves, it may create one or more one-shot or continuous effects. Static abilities may create one or more continuous effects. Text itself is never an effect.
Playing a land causes a land to enter the battlefield from your hand, but it's not an effect.
Vivid Grove has a replacement effect that modifies how it enters the battlefield, but if it's not entering because of an effect, the source of those counters is not an effect. (The replacement effect itself isn't the source of the counters, since it's just modifying another event.)
So if you put
Vivid Grove onto the battlefield with
Walking Atlas, that's an effect, and
Doubling Season applies. If you play it from your hand, though, that's not an effect, so it didn't apply.
But now the rules specifically state that it does apply there, so this bizarre corner case no longer exists.
There was a similar interaction between
Kalitas, Traitor of Ghet and
Doubling Season, where a creature killed by
Terminate would create two Zombies, but a creature killed by damage would only create one. This is also affected by this rule change. (You'll always get two Zombies.)