Hi Ravan,
Good question. Put simply, the use of the term 'visual' in this instance is not a reference to your in-flight conditions but rather an assignment of responsibility for terrain clearance from the controller to the pilot.
Whenever we give a vector below LSALT (normally Minimum Vectoring Altitude (MVA) from our Radar terrain Clearance Chart (RTCC)) to a VFR aircraft or an IFR aircraft in VMC, we need to ensure you are aware that, even though we require you to fly the heading, you are actually responsible for terrain clearance.
Typically you would get this as part of a departure instruction (eg ABC maintain runway heading VISUAL, climb to five thousand, cleared for take-off).
As a technique, I insist on the word visual being read back by the pilot.
Hope that helps. Let me know and I'll post the relevant bits of AIP/MATS.