wpgraft never reads WPBakery’s data. There is no exporter, no importer and no shortcode translator anywhere in this, by design, because those are the parts that break. wpgraft reads the page as your browser rendered it: the real elements and the real computed styles on each one, at every breakpoint you ask for. It then rebuilds that as Gutenberg’s own native elements, with the styles written into Gutenberg’s own fields. Nothing that arrives depends on WPBakery still existing.
This is the migration that has been sitting on your list for years, and the reason it never happens is the shortcode graveyard: deactivate WPBakery and every page fills with [vc_row]. wpgraft never reads a shortcode. It reads the page WPBakery rendered and builds it again out of core WordPress blocks, with no builder, no shortcodes and nothing to deactivate later.