It is not possible to make a backup of forms and it is not possible to export a form on one Joomla! installation and import it on another one. There are two major reasons.
Visforms supports the Joomla! ACL and therefore data, which belongs to a form definition are stored in the Joomla! core #_assets table. That is, how Joomla! manages the ACL feature. These data are individual for a specific Joomla! installation and cannot be transported to another one.
We contiously implement new feature for Visforms which often requires some changes on the Visforms tables in the data base. Update installations manage those data base changes properly. (And downgrading is not possible from a version that comes with data base changes). An export/import script would be highly prone to mess up the data base for example if you run it between different Visforms versions. The effort even to strive to create a script that handles all possible error is gigantic and not worth the benefit and actually we think it is not possible to create a secure scirpt at all. There are simply to many thing which can go wrong.