OFX overlay is a form of control that can adjust OFX directly on the preview window.
Vegas currently only supports point-based overlay controls, which can only provide simple control. But cannot control rotation, scale, aspect ratio, etc like the complete OFX overlay.
The complete OFX overlay seems to need Vegas to update the OFX API to the latest 1.4 before it can be supported. Now OFX API 1.5 is coming soon (https://github.com/ofxa/openfx/blob/master/release-notes.md#release-notes---15-upcoming) .
If Vegas can update to the latest and support the complete OFX overlay, that will make it very convenient for users to adjust OFX directly in the preview window! It also prevents some new versions of OFX plugins with OFX overlay from abandoning Vegas ( for example, Filmconver Nitrate abandons Vegas because of OFX overlay https://www.filmconvert.com/plugin/ofx ).