During fluid deprivation test, the ADH (vasopressin) level dropped down to 1/3 of pre test level and below reference values. Is that a sign of diabetes insipidus?

Not necessarily. The most important thing to be following in a water deprivation test is the urine and serum osmolality. If the serum osmolality rises above normal, but the urine osmolality does not rise significantly, you have diabetes insipidus. Measuring ADH (vasopressin) levels can be unreliable.