So, I went ahead and created an account with Air India with same details as my Vistara account, but I don't see an option to link my account any longer. It appears that Vistara has removed the option to link accounts with Air India
I went through the email that Vistara has sent back in July regarding linking accounts and it says that if I haven't linked after the migration has started, either they will create a new AI account for me or they will merge with an account that has the same details. I have no idea how this is going to play out now!