The Vietnamese sentence is not natural, I don't know how to change it though. Anyone know?
it's not unnatural at all, simply that the speaker put much emphasis on their resolution to not buy at that time.
if you plan to buy but haven't yet: "tôi chưa mua vé".
if you haven't bought and don't plan to: "tôi không mua vé".
I offer a dissenting opinion on the sentence. It is an accurate translation. I tell you I am off to buy a ticket in a reserved section. I tell you when I return, "I did not buy the ticket." There are occasions when you would use "a", too. Both are correct English and commonly used. I have over sixty years seniority in speaking English to back me up, lol.
If we are talking about a specific ticket, we, of course, say "the ticket." The point is that "buy a ticket" is what we say most often and should be the first choice for translation.