if it's a precision approach (ils, mls, par, ...) only visibility is necessary (not talking about operational decision, but jar-ops here), and you can therefore forget about the ceiling (not the case if it's a classic approach).
if you've recieved the atis and it's giving you less than 550m (say 450m) then they should be in lvo conditions and your problem is the aircrafts approach capacity (cat 2, 3 ...).
if you're cat 1 only and the viz is less than the minimum, you cannot proceed with the approach further than the outer marker or it's equivalent position (either a published equivalent ie dme distance or 1000' agl). if you've started the approach because the viz information you have is above your minimum and have gone further than the OM then you can proceed to the mda whatever the viz or rvr giving to you by atc.
hope i've answered your q
seb