It depends upon the exact developer you are talking about. To build a quality product, a Vanilla or React developer should follow all updates and put efforts constantly to perfect the execution of the product. But in any case any developer, who really wants to become pro, should learn both React and Vanilla, and a variety of other technologies to be highly competitive and not to leg behind. In the case both developers grow professionally with the same speed (‘cause everybody has their own speed of learning and producing), they will just fall in love with that what they use, and will be pretty similar in their educative level in this sphere.
At our company every Vanilla and React developer enjoy both of them, they just use them for different cases, but they are well experienced in these two, and not only.