You mean mashing container? That or i am not understanding you...
Brewing container, there are a few things that can go into that:
mashing pot (its ran at around 150F to break the starches in barley and/or wheat into sugars) It's generally simmered, but it's not often under any pressure.
boiling pot - something that generally isn't even covered, its where you boil your malt for a period of time with spices (hops and what have you) to sterilize the malt, kill any wild yeast/bacteria that may be there after mashing (if you mash your own) and to infuse those flavors all together.
Carboy - this is the vessel that has some internal pressure, it's formed by the CO2 that is produced by the yeast as a byproduct of fermentation, some internal pressure is there because the airlock has water in it, and it takes some pressure inside to get excess CO2 to escape. But you cant keep a lower pressure here, nor does it have anything to do with boiling, because the contents go and hopefully stay in the carboy at 55 degrees for lager and around 68-70 for ale.
Now what i think you mean is a still, in which both of what you have said is very true, but we havent talked about anything that needs distilling yet... But it is true, you do want to keep a low pressure in the still to keep special flavors
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