Pokemon Essentials is "officially" for RPG Maker XP only. I don't believe there are any plans to move Essentials to VXAce in the near or far off future, considering that RPG Maker's scripts are coded in RGSS1 (Ruby Game Scripting System), and RPG Maker VX Ace scripts are coded in RGSS3. There are several differences between both languages, and it would require a more than likely full structural overhaul of every script to make it RMVXA compatible.
With that said, there have been attempts to port Pokemon Essentials to RPG Maker VX (And VX Ace) in the past, but these attempts either died down or are in a very slow development, as I don't see much of them anymore on the threads.
Should there be one? While VX and VX Ace have features that RPG Maker XP doesn't support by default (More music formats, loopable .ogg files, unlimited autotiles for a map, multi-tileset support, the quick event generator system), there is no need to spend extraneous time porting over Pokemon Essentials to RPG Maker VX (Ace), especially considering that people working on games would be unable to continue working on them, their entire projects would either have to be continued on an Outdated Engine, or forced to restart as a whole on a new engine. I mean, I certainly wouldn't want to spend months redoing all the progress on my game just to keep up with the shift in engines, but I digress.
Short answer:
Does one exist: Yes, incomplete ones
Should one exist/Should Essentials convert to RMVXA: No, too many complications involved
Can you convert a XP game to VXA: No. It is not possible due to changes in the scripting engine.