The difficulty in doing that is that the very act of pouring the vermouth out of one bottle into another will oxidize it.
Yes, I feared that would be the case - but will the resulting damage be less than repeatedly opening a large bottle, rather than this procedure of opening once and then resealing? Is using a vacuvin or similar a possibility, or is that pointless, too?
i'd say these vermouths are less parishable than you think... pouring a vermouth into a new vessel will technically oxidize it but you can minimize the effect down to a negligable level.
table wines are racked and exposed to air all the time for brief periods and they come out just fine. i separate my liters of sweet vermouth into tiny canning jars and find no ill effects. i fill them to the vary brim and use a spoon to put them into my oxo jigger when i'm ready to use them. some wine makers warm their wines to slightly above room temp because that way the act of pouring will dissolve less oxygen into the wine. a normal bottle of wine takes many hours to absorb its terminal level of oxygen that sets the ball rolling on spoilage so the mere seconds of racking at a high room temp into a vessel filled to max capacity should be no problem. i endorse the tiny canning jars. they are hard to pour from but if you are patient and at home a spoon works fine.
any high volume bars using 375's that want to be really anal should consider that they don't know the born on date of their vermouths and they could be trapped in the distribution system for a while (i see two or three different label styles at many liquor stores from any one brand). if a bar really uses alot they may get better performance out of liters or bigger because they age slower than the 375's.
sweet vermouths have the lowest amount of alcohol, maybe comparable to dessert wines, and its known that dessert wines age very fast in their early life and then even out to a slow crawl... i can't imagine sweet vermouth would be too different. using all the wine rules of thumb, a larger size would decrease that effect...
if i did serious volume and was really anal, i may even buy handles of vermouth to decrease the effects of the distribution system then decant it all into small bottles for all my stations and inert gas the left overs...