Hi ludovicbs,
As of now, we do not have any updates on this when our VDDK team has planned for RHEL 8. Probably, with major upcoming releases, you may find the official support on RHEL 8.
Also, we highly value all customer feedback and input in relation to our products and have provided a direct channel for you to submit your feature request on our public website, it can be accessed at http://www.vmware.com/contact/contactus.html?department=prod_request