Here's what I found out: RVR readings are taken using a transmissometer (machine), and VIBAL (visibilité balise) is a RVR/visibility reading taken by a person.
As too how Pilots can use it legally is another thing. Any ideas?
RVR is RVR, regardless of how it is assessed. Commonly it is measured by transmissometer and you will often hear this referred to as IRVR, standing for Instrumented Runway Visual Range. The other common method is to count the lights, often referred to as Human Observer RVR or HO RVR. Both are standard methods (there are others) and assessments from whichever method is in use will be passed to pilots as THE RVR for a runway. I've never come across a pilot that needed to know the method of assessment - or cared, come to that.
By the by, IRVR isn't completely straightforward nowadays. IRVR used only to be assessed by transmissometers - for the last 10 or 15 years forward scatter meters have been in operational use. Again, as a pilot you'll probably not know if such a system is in use, you'll just be passed an RVR value.
HO RVR is acceptable for Cat I operations but, if I recall correctly, for Cat II/III operations RVR must be assessed by instrumented means.
If you really want to know more ICAO Doc 9328 Manual of Runway Visual Range Observing and Reporting Practices will probably keep you rivetted.