"I sell computers."
Translation:Tôi bán máy tính.
The longer is the traditional, the older way of naming „computer“, it is replaced in practice by a shorter form without „vi“. It's like two writings for the same word, like programme/program in English.
There are some other names of „computer“, I suppose; see the article https://vi.wikipedia.org/wiki/Máy_tính (at least it is named máy tính, and that tells us which form is most common).