"Particular" here just means an individual example rather than one with features that sets it aside from the norm. A Thermos is a vacuum flask in the form of a commercial product.
The big OED gives this definition:
"A registered trade term noting a flask, bottle, or the like capable of being kept hot or cold by the device (invented by Sir James Dewar) of surrounding the interior vessel with a vacuum jacket to prevent the conduction of heat. Hence (freq. with small initial) applied loosely to any vacuum flask."