I am surprised no P11 gurus have chimed in, so I will contribute the little I know.
If the P11 oil tank is similar to an Atlas, in function, not shape, the forward facing tube is for a vent hose to the crankcase breather. I don't know if the '67 model was still using the timed rotary breather, but if so, that is where the other end goes.
The rear facing tube is the oil tank vent. This goes to atmosphere via a tube. On my Atlas, this tube ends at the front end of the rear chain guard. On the P11, it may drop down to vent near the front of the rear tire. I have seen some vent tubes routed along the inside of the rear mudguard, terminating at the end of the mudguard.
I assume you mean a PCV (not pvc), or positive crankcase ventilation valve. There is no problem fitting one of these between the crankcase and oil tank (free flow towards oil tank).
If your engine has the timed breather, and if a PO has not removed the spring and vane, there will be no advantage to fitting one. I have measured the crankcase pressure with an XS650 one way reed valve in series with the rotary breather, and found no difference in crankcase pressure. Otherwise, go for it .... it will not hurt anything.
Slick