NVLD is still a highly theoretical entity, as far as I know. There remains much subjectivity within it. But it is a phenomenon which is well-documented, but imperfectly defined.
Psychology is not exactly an exact science. Diagnoses are always in flux, as new research findings are obtained. NVLD should be taken seriously because it is a recognized entity. There are always differences in how "experts" perceive such things as NLVD and Aspergers--but there seems to be more consensus as to Aspergers.
Autism--especially classical autism--is much better defined, and much more consistent as far as symptomography is concerned.
To say that even autism is a "medical diagnosis" with the same objectivity as, say, pneumonia, would, in my opinion, be an erroneous statement.