Enterprise vault for Distributed exchange environment

It depends on how large your environment is, how much data you already have going over the pipe etc, I would recommend checking this out, then thinking about how much data is likely to travel across the link with EV installed and see if you are going to come anywhere near saturation or not.

There are so many variables involved it is almost impossible to say one way or another from where we stand, trying to give advice on someone else's environment without knowing the full ins and outs is almost impossible unfortunately, without sounding funny you are more than likely the person in the best position to do that.