Only the CMD can fly it.
It's not only a matter of being able as an FO. It's also to time available for a CMD to correct if it should be necessary. Since it's still the responsibility of the CMD, he also has to have the right to intervene. On a tight approach like LCY, there might be too little time available.
You might be also correct that you could train most FOs to the task, but in more difficult situation, it's still "best use of equipment" to let the CMD as the PF. Otherwise you could also argue that an FO should do the Cat III, the OEI, the flight control failure... - the list goes on.
As for your question about the Vref I didn't really get what you are asking.
hth,
Dani