Yep, PVC does emit a gas from the plastic. The gas is the plasticizer that is used in its manufacture and the emission of this gas is what causes the plastic to eventually become brittle.
A good coat of epoxy paint will reduce the gassing by a fair bit but it tends to continue.
The plasticizer won't destroy the aluminum coating but it will cover the mirror and you will have to do regular cleanings to remove it.
I will note that if you don't like this, Hastings Irrigation Pipe (on the web) does have some nice aluminum pipe in a special astronomy section of their website that isn't too expensive and will do very nicely for you.