I have generic functions on my Vue JS (v. 2.5.17) project, but I don't know where should I placed them, there is any convention like in Rails helpers ? Or can I create a file anywhere and just import it where I needed ?
You are looking for either Plug-Ins or Mixins.
Use mixins if you want to add the same functionality for multiple components. Mixins basically add methods from the mixin to those of the component using it.
Use Plug-Ins to add global level functionality.