The Vs. animation depends on the trainer type of the enemy trainer. So long as the appropriate graphics exist in the folder Graphics/Transitions, it'll play automatically.
The required graphics are vsBarX.png and vsTrainerX.png, where X is the enemy trainer's trainer type. It does not check for any other graphics it uses, including the player's graphics (which also use the trainer type), but they already exist in Essentials - you may just want to replace some of them.