The first question is why the user must be root if he want to collect hardware counter event?
The second question is why only one process can use Vtune to collect hardware counter event at the same time?
I have used Vtune to profile the cache usage of two different processes simultaneously, but failed.
You do not necessarily have to be root to use event-base sampling (EBS). However, you need to have access to the sampling device. During installation of VTune, a group is selected that has access to the sampling device. The default is the group "vtune". You can add your user to this group and then (after logging in again) use VTune as this user.
Furthermore, you can sample more than one process. Simply choose that you want to monitor the whole system when setting up your project. You can still filter out the events of an indivial processes when analysing the results.