The reported vv would serve as a ceiling in my world, and should be above your required ceiling (for non-precision). But vv is used when ceiling is typically below 500' and when it's difficult to judge because of precipitation.
From my personal experience, ATC is not taking the decision for you, which sometimes would be appropriate in my view. For instance you're doing a CATIIIa approach at night, RVR is 350 and then the touchdown zone lights fails. The controller should both know that the required RVR is then 550, and more importantly advise every aircraft of this ("..below minimums.."). I get the feeling they just leave it to us to decide, 'cause they won't get in trouble anyway...
ATC is there to help us out, but I have noticed that they can sometimes report wx to be at minimums just to give us a chance. Especially when it comes to wind, which is very difficult to report because it changes all the time when it's gusty and close or above limits. It's not much of a help when you get a report of max x-wind and you're not able to align in the flare.