The armour values are not really analogous as not only did the rules for vehicles change significantly but the armour penetration of weapons also changed significantly.
In 2nd Ed any hits on vehicles were randomised to see what part of the vehicle was hit (tanks were hull, turret, sponson, and or tracks, walkers were legs, body, gun and or driver). Each part then had an armour value for front/side and rear that you needed to beat with your weapons AP rolls, then if you managed to beat the armour you rolled on an injury table for that vehicle part to see what happened.
Guns in second Ed also had bespoke armour penetrations that were made up of static numbers plus 1-6 various dice from d3 to d10’s. Autocannons were d6+d10+8 for example.
Third Ed basically streamlined all armour values back to between 8-14 and all weapons AP was basically strength plus d6.
Third Ed also replaced all the bespoke damage tables for each vehicle with a single universal set of tables using glancing hit, penetrating hit and ordnance hits.
Have a read through
This article to read about some of the changes in a bit more depth.
Honestly though if you just want a quick conversion then take the second edition hull values subtract 10 from them, divide that result in half (rounded up) and add 9. It will put you somewhere in the ball park.
