Is vaadin compatible with my lit-element components? and with what versions?
You can use any Vaadin version that uses npm and JS modules, i.e. Vaadin 14 or newer. With Vaadin 14, you cannot use the compatibility mode that uses Bower for package management and HTML imports as the module format.
As a practical example of how LitElement components can be used, you can have a look at https://vaadin.com/labs/lit-element.