Ah, found this from New Oxford American Dictionary: Usage: The preposition 'upon' has the same core meaning as the preposition 'on'. Upon is sometimes more formal than on, however, and is preferred in the phrases once upon a time and upon my word, and in uses such as row upon row of seats (definition 2) and Christmas is almost upon us (definition 1).

http://dictionary.reference.com/browse/upon. Has some good examples. —used to emphasize something that is repeated many times------ this seems wrong and I cannot think of such a context. —used to say that someone or something is very close or has arrived ----- 'he crept up on me' is the closest phrase.... But that is using two words 'up' and 'on'....not 'upon' Time for a better dictionary? ;-)

"Upon" is also used in this way in the example you cited ("He carefully placed the vase upon the table"). It is usually done for literary flourish or poetic tone. It essentially means "up on" or just "on". In your example, using "upon" has the effect of emphasizing the "carefulness" of the way he was placing the vase. But the sentence is also correct just saying "He carefully placed the vase on the table". Another example: The little boy rested his head on his mother's lap. = The little boy rested his head upon his mother's lap (sounds a little more poetic/dramatic).