vRA does not require a separate cluster for vRA managed machines. If you only have a single cluster vRA will still recognize it, allow you to add it to a fabric and provision to it. However, I would separate them if you are able for a 2 major reasons.
1) vRA does not recognize your non vRA managed workloads when you are making reservations. So if you have 200GB of Memory in your cluster and you have 100GB being used by non-vRA managed machines vRA will allow you to make a reservation for the full 200GB and provision to that. When machines are created they will only count against whatever you created in your reservation and you will have to combine that with what vCenter is reporting to see utilization. So it is very easy to get yourself into trouble depending on how you manage over committing resources. Especially with storage.
2) I find that I often end up with vRA exclusive networks, templates and storage. Again, this becomes easier to manage when it is all in its own cluster. If you have users accessing vCenter that aren't familiar with the vRA setup they can move something out from under vRA and cause all self service provisioning to grind to a halt until you put it back. I try to create that cluster and keep as many users (admins included) away from it.
Like much of a vRA it comes back to how you want to manage and organize things.
