Hi,
First: Vienna Ensemble (Pro) is not a player. It is a Instrument host that can offload the instrument hosting from your DAW and even to slave machines in your network. You can add VST effects and mix and route everything to your DAW. Also Vienna MIR works best within VEP. It's a very cool tool and can optimize your template and workflow.
Vienna Instruments is a sample player of the silent-stage instruments from VSL. It works great, is performant and the Pro version adds some great additional features.
Synchron player is the newest sample player and is used for the synchron series instruments. It has a different approach: articulations are organized with a tree-structure instead of multiple matrices like in Instruments. Also it has much better support for multi-mic sampled instruments.
In short: If you buy an instrument from the VI series, you will use the Instruments (Pro) player, if from Synchron Series you will use the Synchron player.
And on top of that whether you use VI or Synchron or both, VE Pro is a nice addition.
Hope this helps.
Best, Ben