Requested calculation, the energy required to vaporise a tank, i'll just use the M1 Abrams as the base model for this calc 


An M1 abrams weighs around 62 tons or 62000 kilograms 

So weight is 6.2e+7 grams

Density of steel is 8.05 g/cubic centimeter

Volume = mass/density 

6.2e+7/8.05 = 7,701,863.35404 cubic centimeters


Value for vaporisation is 25700 joules per cubic centimeter

7701863.35404 x 25700 = 197937888199 joules

Converting that to tons of TNT gives us a value of 47.3 tons of TNT

Final Result

Vaporising a tank : 47.3 tons of TNT (City block+)