We don't have a direct compare tool, but we do have a newer feature which can help get you there!  In VistaDB 6--and in the later VistaDB 5 versions (introduced back in 5.2)--Data Builder has a feature in the SQL Script menu to Export Schema as Scripts.

This creates a set of individual script files for each object organized into folders (such as "Tables", "Views", and so on) under a targeted root directory.  It's particularly designed to be useful with a file-tree compare tool such as Beyond Compare or to be dropped into Git source-control and tracked as versions change over time (you do have to manually initiate the export from a "current" database whenever there might have been a schema change, of course).  Then Source Tree or other Git-based versioning UI tool's comparison can show you the differences on an object-by-object basis.

You can also try SQL Script -> Generate Schema Deployment Script and then use a single-file-pair comparison tool (if it includes the unchanged parts of the file for context; otherwise, it could be hard to tell what object a localized change is in).

We don't know of a tool (haven't had someone point it out to us).  We have talked to the Red-Gate folks a few times about figuring out how to sync between SQL and VistaDB but we have a lot of work we'd need to do to implement SQL Server's schema query and modification syntax.