Page 93 of Tactical Operations: advanced Units and Equipment states that
"Penetrating critical hit checks (including “floating” critical hits) against a unit protected by Hardened Armor (such as those caused by rolling a 2 on the
Hit Location Table, or during a successful Swarm Attack) suffer a –2 modifier to all rolls on the Determining Critical Hits Table (including Vehicle Critical Hits
Tables)."
the question I have is if this -2 modifier is additive to the -2 modifier inherent to Critical Hit Checks made when Vehicles are struck by Inferno Missiles (as described by page 142 of Total Warfare