Странно, что тут не написано, но они зависят от реализации Project Valhalla, и пока он сам не выйдет из превью, Vector API тоже не выйдет:

> Alignment with Project Valhalla — The long-term goal of the Vector API is to leverage Project Valhalla's enhancements to the Java object model. Primarily this will mean changing the Vector API's current value-based classes to be value classes so that programs can work with value objects, i.e., class instances that lack object identity. Accordingly, the Vector API will incubate over multiple releases until the necessary features of Project Valhalla become available as preview features. Once these Valhalla features are available we will adapt the Vector API and implementation to use them and then promote the Vector API itself to a preview feature. For further details, see the sections on run-time compilation and future work.

Короче говоря, ждать придётся **как минимум** столько же, сколько value-классы из Project Valhalla.