The VGS Proxies alias data based on what the data to be aliased is. For instance, you have a credit card number 4111111111111111, and alias it, you will get an alias back like tok_live_a1b2c3d4e5f6. That token that has been created is now statically mapped to the aliased value of 4111111111111111. Any new or different values to be aliased will always return a different alias. At this time, you can only use a VGS alias with the proxy or Vault API to reveal the aliased value, or if you wish, to delete it. Because aliases are mapped statically, there is currently no function to use an alias to update the value tied to it.
So, if you wish to update the card number you have saved in your database, (say, to 4444444444444448), you will have to alias the new card value, and save the new alias in it's place. It is not possible to modify the value of an existing alias and keep that value tied to the same alias.