I don't think a “ranking of vulnerability” is, generally speaking, a term that the international community would use. There are seven identified specific vulnerabilities. How they end up weighted against each other is very much case by case, and I think that's probably the way it has to be.
To speak hypothetically, you could have a member of a religious minority in a camp who's at immediate risk. You could have an unsupported female with a couple of dependent children who could be at immediate risk. You could have a member of the LGBTI community in the refugee camp in Kakuma whom UNHCR is simply not able to provide protection for, and who needs to be helped immediately for resettlement.
It is really case by case how it unfolds in these situations. It's very challenging work in a situation where, unfortunately, the system can only resettle a small fraction, even of the most vulnerable refugees.