This functionality is experimental and may be changed or removed completely in a future release. Elastic will take a best effort approach to fix any issues, but experimental features are not subject to the support SLA of official GA features.
Currently Vega version 4.3 and Vega-Lite version 2.6 are supported.
In Vega it is possible to load data dynamically, e.g. by setting signals as data urls. This is not supported in Kibana as all data is fetched at once prior to passing it to the Vega renderer.