To write a detailed summary, whether it is of a poem, a chapter, an essay, or a book, we want to fist pick out the important details in the piece. It may be a little more difficult to summarize a poem as we must first understand the meaning and theme/themes of the poem, but not impossible. Below are a few ideas to help get you started.
Let's take a look at the first line as it is actually very revealing about the content of the poem: "When the gong sounds ten in the mourning and I walk to school by our lane." What does this one line tell you about the main speaker/character of the poem? We know that the speaker is walking to school; hence, the speaker must be school-aged, right? The image of the gong sounding also paints a very revealing image. Where could the speaker be that a gong sounds in the morning? The speaker is certainly not in the US. But if we look at the poet's biography, we learn that Rabindranath Tagore was from Calcutta, now spelled Kolkata, India; hence, we can deduce that the speaker is school-aged and also lives in Calcutta, just like the poet. It may even be fair to say that the speaker is a reflection of the poet. Therefore, from just this one line, we can summarize that the poem is about a school-aged boy or girl who lives in Calcutta.
The next line is also important as it establishes the poem's whole underlying theme. The next thing the speaker describes is a "hawker," meaning a street vendor, who is calling out his wares, which happens to be "crystal bangles." The speaker sees the street vendor's liberty as he has "no place he must go to," like school, and no one demanding that he come home. Due to what the school-aged speaker imagines is the street vendor's liberty, the speaker imagines that he wishes he was a street vendor rather than a school-aged boy or girl. Throughout the rest of the poem, the speaker makes similar comments about the liberty of other grown ups he or she sees, like the gardener and the night watchmen. Hence, all in all, this poem reflects on a young person's eagerness to grow up, get a vocation, and experience what he or she perceives to be an adult's freedom. So if you were to continue to summarize the poem, you would continue to describe all of the vocations the speaker observes and how he or she sees the adults with these vocations as having more freedom than a school-aged person.
I will attempt to answer your question with an introduction to Tagore, one of my favorite writer-poets of all time. He was an ardent nature lover, who took the time to linger with the flowers and the leaves, to employ a literary cliche.
He is also most known for being an avid observer of the world through clear eyes, especially of human nature. Having been born in a part of India that is even today known for its patronage of the arts, he was particularly defiant of religious oppression and patriotic fervor that compartmentalized people by nationality instead of arising from a sense of pride of/in their motherland.
Here I present to you one of his quotes about his staunch refusal to categorize humans by race, creed and religious leanings:
"Patriotism cannot be our final spiritual shelter; my refuge is humanity. I will not buy glass for the price of diamonds, and I will never allow patriotism to triumph over humanity as long as I live. I took a few steps down that road and stopped: for when I cannot retain my faith in universal man standing over and above my country, when patriotic prejudices overshadow my God, I feel inwardly starved."
The poem 'Vocation' is an extension of this very same "refuge in humanity" - a celebration of the everyman as seen through a young schoolgoer's eyes. The child is seeking release from the prison of conformity as imposed by parents and wishes he could rather be free like the hawker, the watchman and gardener that he sees on his way to and from school; men who are free to choose their tasks without feeling the walls of time closing in.
The innocence of the child is juxtaposed with the harshness of reality of the lives of these menial workers, but the child can only see the 'freedom' they enjoy while he has to be satisfied with seeing them through the bars of his window. The window becomes mimetic of his sense of feeling shut away from the world outside. Instead of sticking to a time schedule as imposed by his mother, he wishes he were outside aping the three men in the wide open spaces that is the world outside his window.
I hope I have helped answer your question in some measure.