The VDDK init image field of the VSphere provider is not mandatory but is required  by default.
Skipping the field will significantly  slow the migration since it will slow transferring virtual disks from the provider.

Current status
That info is currently represented as a recommendation text message within a Patternfly Hint in create/edit provider pages and in tooltips:

Expected status
Users might miss this info so need to put the recommendation to use VDDK within an alert warning section and rephrase the recommendation to emphasis..