There are users of WC Vendors Pro and WC Vendors who use the Flatsome theme, as I have worked on some of the sites. The theme seems to work fine with WC Vendors. You can definitely run a WC Vendors Pro based marketplace with the Flatsome theme.
The BuddyBoss Marketplace plugin is entirely separate- built specifically for use with the coordinating template OneSocial. If you try to use that plugin without their template, it will throw errors and also totally mess up the layout since there will be doubling-up of templates and actions using the same hooks. We don’t build the Marketplace plugin, and the Marketplace plugin is absolutely *NOT* in any way necessary to run WC Vendors or WC Vendors Pro. It is built by BuddyBoss to work with their theme and our WC Vendors plugins, and buddypress plugin– to make a social marketplace using their theme and our plugins.