Yes, but no. No, in that these days, GPUs are entirely scalar from the point of view of invocations. Using vectors in shaders is pointless - it will be as fast as scalar variables (double instruction dispatch on AMD GPUs is an exception).
But yes from the point of view that a collection of invocations all progressing in lockstep get arithmetic done by vector units. GPUs have just gotten really good at hiding what happens with branching paths between invocations.
But yes from the point of view that a collection of invocations all progressing in lockstep get arithmetic done by vector units. GPUs have just gotten really good at hiding what happens with branching paths between invocations.