1. VAO is not the competent authority to issue a death certificate.
On an application to the Tahsildar to issue a death certificate of the deceased with all supporting documents and reasons for not registering the death at that time, the Tehsildar will issue instructions to the Revenue inspector to enquire the same through VAO and submit a report, after that the Tehsildar will issue a certificate opinion that the death might have taken place during that period which is revealed in the enquiry.
Then on that basis the applicant can file an application before the District execution court seeking instruction to direct the concerned registrar of births and deaths to register the death and issue a death certificate as per procedures of law. Any certificate obtained without following the said procedures will be invalid and null and void. As you have applied for information through RTI act, you may wait for it and in the meantime you can challenge the death certificate ion the basis of the procedures as explained above.
2. If this property was A's self acquired property and the same was transferred by a testamentary disposition, then other than the beneficiary of the Will nobody can claim any share in the property as a right. The son of deceased A is doing it with an intention to grab the property or at least to extort huge money from you on the basis of this non maintainable civil dispute, hence you can very well challenge the same on merits and get it dismissed. A has transferred this property by a registered Will and it was properly acted upon by the beneficiary, besides the son remained silent without any challenge up to three years after becoming major hence any act done beyond the limitation period to challenge the Will or making a stake to claim a share in the property after the said limitation period is not maintainable
3. you must wait and watch the proceedings of the case patiently, don't jump into any hasty conclusion about the outcome of the case. .