The word you're probably looking for is ampule.
Ampoule
ampoule
A food steamer or steam cooker is a kitchen appliance used to prepare various foods in a sealed vessel that limits the escape of air or liquids below a preset pressure.
vial
Autoclave
r67
Autoclave
If the vacuum pump is on, the water will continue to boil. If the vacuum pump is off as the vessel is sealed, the boiling will stop at a point of equilibrium.
Liquids have a definite measurable volume but no definite shape. They assume the shape of whatever vessel the liquid is poured into.
I guess it depends on whether it's sealed or not. If it's open to the atmosphere how can it be under pressure ? If it's sealed it could be put under (positive or negative) pressure so would be, by definition, a pressure vessel IMHO.
Beakers hold solids or liquids that will not release gases when reacted or are unlikely to splatter if stirred or heated.
an obs. form of began, imp. & p. p. of Begin, sometimes used in old poetry. [See Gan.], A drinking cup; a vessel for holding liquids., A vessel or case of tinned iron or of sheet metal, of various forms, but usually cylindrical; as, a can of tomatoes; an oil can; a milk can., To preserve by putting in sealed cans, To know; to understand., To be able to do; to have power or influence., To be able; -- followed by an infinitive without to; as, I can go, but do not wish to.
After a dipper. From the Wiktionary: "... 2. A cup-shaped vessel with a long handle, for dipping out liquids."