I haven't seen this issue raised previously but (a) it is likely to be increasingly common, (b) appear on the face of it to present unique problems, but (c) actually be accommodated reasonably well within existing guidelines (e.g. American Psychological Association style, MLA, Chicago, Vancouver).
It is probably most useful to give a specific example. Unfortunately, I could not easily find a suitable medical-sciences paper that, itself, used Vancouver style ... but I did find a paper in American Historical Review with the following basic information.
Here is some additional information about a specific open review:
Vancouver
Now pretending that it is a medical-sciences paper using Vancouver style ...
For the review:
[Reviewer's Last Name, First Initial(s) if available]. *Reader's Report #1 (Second report)* [Internet]. The American Historical Review. [cited year month day]. Available from: https://ahropenreview.com/HistoryCanBeOpenSource/readers-report-1-second-report/
For the final revised manuscript:
Author(s) Last Name First Initial(s). *History Can Be Open Source: Democratic Dreams and the Rise of Digital History* [Internet]. The American Historical Review. [cited year month day]. Available from: https://ahropenreview.com/HistoryCanBeOpenSource/second-revised-draft/
In Vancouver style, you'd likely include "[cited year month day]" with the date you accessed the web page. Also, the author(s) name and details should be provided as they appear in the source. If multiple authors are present, list them in the order they appear in the publication.
APA
For the sake of completeness, given that my example is actually from the humanities ...
For the review:
[Reviewer's Last Name, First Initial(s) if available]. (Year).*Reader's Report #1 (Second report). In *The American Historical Review*. Retrieved from https://ahropenreview.com/HistoryCanBeOpenSource/readers-report-1-second-report/
For the final revised manuscript:
Author(s) Last Name, First Initial(s). (Year). History Can Be Open Source: Democratic Dreams and the Rise of Digital History. *The American Historical Review.* Retrieved from https://ahropenreview.com/HistoryCanBeOpenSource/second-revised-draft/
Note: Your should replace "[Reviewer's Last Name, First Initial(s) if available]" with the actual name(s) of the reviewer(s) if provided. If the year of publication is not explicitly stated on the webpage, use the access year or an approximation. Again, although I could see indicators of an approximate date, the web page doesn't seem to say when it was published.