From which multi vendor plugin you had migrated?
I believe you have migrated from WC Marketplace (WCMp) multi vendor plugin.
Certainly we have withdrawn compatibility with this plugin around 6 months ago due to it’s uncontrollable bugs. https://wclovers.com/blog/were-breaking-up-with-wcmp/
I am really sorry for your trouble but we are quite helpless as WCMp new version vendor orders are not standard WooCommerce order so it’s not possible to migrate those.