We recommend that you work with an Azure consultant to see which of their servers are the best to choose for Vault. This selection is not easy because it also depends on a number of different factors. How many users? How many files are created per day? Where are the users located? What type of user are they? What edition and release of Vault are you installing? What amount of growth do you expect to see over the next 3 - 5 years?
Since you are moving from an on-premise version of Vault to Azure, you should start with the hardware you currently have and see how that matches up to what Azure has to offer.