Since we needed it a lot, we decided to develop the Chip-ComboBox:
This enables the user to select one or more elements from a drop down menu. We focused on usability for the end user and readability for the developer.
You can use it right out of the Vaadin-Directory or even add you own changes since it is free and OpenSource: GitHub - xdev-software/vaadin-chip-combobox: A ComboBox with Chips/Chip Components for Vaadin Flow.
Enjoy! 
Johannes
XDEV Support