I'm hoping someone can help me define my vessel.

I have a reservoir that holds a fluid that is not hazardous at temperatures below about 300C.  The fluid is basically on the saturation curve so you can pretty much control the pressure by its temperature.  It will have some air which will affect things but not significantly.  It is intended to be used in a controlled environment such as office space.

The operating pressure is usually around 7 psia, yes it is a vacuum.  The product it intended to be shipped without fluid.  The specifications for the system indicate a maximum storage temp of 55C.  This would put the reservoir pressure still below atmosphere.  At close to 60C you hit atmospheric pressure, 80C would be around 30 psia or 15 psig.  The device has a single 10 psig PRV.

Could this ever be defined as a pressure vessel?  Even though we intend to ship it without fluid and the specifications limit it to 55C storage temp we can't control what somebody does to it after it ships.  Proof pressure of 150% of operating pressure is usually interpreted as psig but is that the right way to look at it.  With a 10 psig PRV one could define that as MAWP then the proof test would be 15 psig or technically should it be tested at 1.5 X 25 psia?

In summary my questions are:
-Is this defined as a pressure vessel (dimensionally it is about 5 high by 15 wide by 30 long)?
-How does the inability to control other peoples actions once the product is sold impact its worst case conditions?
-Should one consider absolute pressures or guage pressures for calculations?

In a nutshell what would you guys use as the proof pressure?  What would you use as the burst pressure or would you define a burst pressure?