Why are you guys making it complicated? Buy a concession myki and you don't need anything else. And don't touch off if you are travelling by bus.
You need a VPT Concession Card in order to be entitled by a concession myki. It's a little card with your passport/school photo on it. If the ticket inspectors notice that you don't have a VPT Concession Card along with your concession myki, then they can fine you.
If you're using myki money, you should touch-off, or you'll be charged a default fare. If you're using a myki pass, like a weekly or a yearly, you don't need to touch-off (but it doesn't matter if you do, it doesn't change anything).
The only time you don't need to touch-off when using myki money is on a Zone 1 Tram.
(I've been using myki for three years so trust me on this)