The World Health Organization (WHO) has a number of apps, I felt I should keep the question rather general in order to prevent people from attempting to close this question for being too narrow, but I really am interested in how I would cite the diagnostic criteria in ICD-10 in Vancouver style.
As Daniel commented, "there is nothing special about The WHO." As with any source, cite in accordance with the citation style you are using. For Vancouver style, two online guides are here and here. Find the section relevant to the source you are using, (electronic book, print book or website), and use the recommended format.