This will almost certainly change once the south concourse is open. The VIA concourse will act as a quick shortcut to Bremner from the Great Hall.
You'll go down the ramp, through the concourse and into this space with Union Plaza, Scotiabank Arena and Bremner straight ahead and the Bay and York concourse on either side:
The wall that separates the VIA concourse from the south concourse appears to still be there in this rendering but there's little reason to keep it or at least remove the doors.
My hope is that VIA is waiting for this to be complete before opening their concourse to the rest of the station. The heavy doors in place look to be hanging on for dear life and awaiting replacement or removal. There's also a staircase at the south end of the VIA concourse that leads down to the lower concourse and food court. Those stairs have been confirmed to be temporary and rebuilding them will directly affect the south wall of the VIA concourse, which is why I think it'll be opened up and better connected to the south concourse.
Since I'm here, I thought I'd do a little photo tour of what I just described above.
When going down the ramp from the Great Hall into the VIA Concourse....
...towards the end you come to this wall:
It clearly looks temporary. If you go around to the corridor behind it that connects the Bay and York Concourses, you'll see what's behind the bump out wall:
Follow this and it leads you to a staircase:
Go down the staircase, and you'll find yourself here:
That wall will come down and will lead you to the lower part of the South Concourse which will exit out of the Union Market door you see here:
Notice how this door is directly below the VIA Concourse so the temporary bump out staircase will no longer be necessary since the spaces will be connected by stairs in this new South Concourse.
Back on our tour at the bottom of the staircase, if you turn around, you'll see that the temporary staircase that forms that bump out in the VIA concourse, has split the stairs in half.
Walk down these steps and you're in the Union Market lower concourse:
This all to point out that this staircase is indeed temporary and that removing it will also take down the VIA Rail's south wall into the new South Concourse. That outer wall can be seen here and indeed does look very temporary:
So when this is all said and done, going from the Great Hall through the VIA Concourse will be the most direct route through the station to Bremner which I think will mean the VIA Concourse will get a lot more through traffic.
Thank you for coming to my TED Talk.