OK, but OP’s request would (in my opinion) be met by the previously requested feature “Customizable vault item types”. Thus, I am suggesting that the current thread be merged with the “Customizable vault item types” thread (not the “Additional item types (pre-defined)” thread).
For example, with this feature, you would be able to create a custom login item template, which would look like the current login item, except that it includes a custom field for telephone numbers. Thus, in every new vault item created using this custom template, there would be a telephone number field available to complete.