The current Vaadin versions, 10-13, uses HTML imports and are distributed via webjars (which are fetched and built from Bower). This means that all web components that you use together with Vaadin 10-13 needs to also use HTML Imports to work together.
HTML imports was part of the initial web component specification, but the web community as a whole didn’t adopt that specification. Therefore most of the market migrated over to ES Modules, and npm. HTML imports and bower goes hand-in-hand as does ES Imports and npm.
Vaadin’s components are built as both: if you take them from bower you get html files, if you take them from npm you will get js files.
For Vaadin 14, we are looking into adding support for bringing in web components as ES Modules directly from npm, which would give you the option to skip both HTML Imports and WebJars. Which one you use would be up to you, but you can’t mix and match where for example the Vaadin components come from HTML Imports and Ionic components from npm. You have to use one technology and stick to that. We will recommend the npm/ES Module way for all new projects, but we will keep the bower support intact so that existing project will still work in V14 and migration from bower to npm would be easier. 14 release date is in three months, on Jun 5 2019, and it is a long-term support version.
So to answer your question: No, you can’t use npm components like the ionic set today. You can use non-Vaadin components that are built on top of HTML Imports, like the ones in [Directory under “Polymer 2”]
(https://vaadin.com/directory/search?framework=Polymer%202) or available in bower. You will be able to take npm components into use in three months if you create a new project or convert your existing project to use npm.