How to create a ISA reference in Vertabelo?

There is no such type of reference in Vertabelo. Please notice that Vertabelo works on physical database model. ISA reference is usually related to conceptual database modeling.

To model inheritance in your database you can use plain references with techniques described here:

http://stackoverflow.com/questions/190296/how-do-you-effectively-model-inheritance-in-a-database

http://stackoverflow.com/questions/386652/techniques-for-database-inheritance