Gameplay is quite simple: just click each node of the tree with the mouse when highlighted, beginning with the base and moving up sequentially to the top of the tree. Each node will stay highlighted for only a short time before dropping back down towards the ground, so timing is important. Clicking in the center of each node will keep the tree balanced and growing straight up, while clicking to the right or left of center will cause the tree to lean in the opposite direction. The objective is to see how high you can make the tree grow before it tips too far towards either edge of the play field.
What is most unusual about Vanilla is its very narrow play field and a potential height that stretches well beyond the height of any browser window. As the tree of Vanilla becomes very tall, it becomes necessary to scroll the browser window in-between clicks, and timing becomes a factor that increases the difficulty.