Although the word 'upon' has the same meaning as 'on', it is usually used in more formal contexts or in phrases such as 'once upon a time' ( used at the beginning of stories) and when we want to emphasize that there is a large number or amount of something ' thousands upon thousands of letters'. We should say 'There is a vase on the table' since we are talking about something touching or forming part of a surface: 'The books were lying about on the floor.' 'The names on the list are in alphabetical order.' 'There are many papers on the desk.' 'She touched him gently on the arm.' 'The file is on the table.' 'She had a puzzled look on her face.' We use 'AT' - to say where something/somebody is or where something happens: 'I met her at the hospital.' 'I'm at home.' - to say when something happens: 'We left at 2 o’clock.' - to mean 'in the direction of or towards somebody/something: 'What are you looking at?' - used with adjectives to show how well somebody does something: 'I’m good at French.' - to state the age at which somebody does something: 'She got married at 25.'

Learner is entirely correct. "The vase is upon the table" just sound silly. The instances where you can actually use 'upon' in cases like these are far and between. Usually only when you literally mean both 'up' and 'on,' said of things in elevated positions, like: "You can see an antenna upon every roof in the neighborhood." As for the rest, 'upon' is mostly used metaphorically, like: "Death was upon him." Or to denote the occasion of something, like: "She was happy upon seeing her grandchild again."

"There is a vase on the table" The vase is "upon" the table... if it were 'at' the table (like a person), that would mean it would be near the table, maybe on a chair but not actually upon the table.