Pemberley, the main evidence all comes from one group, who have now done several trials of increasing size, all looking just at convergence insufficiency. Hardly overwhelming endorsement for the research junkies amongst us, but OK.

http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2821445/
http://www.ncbi.nlm.nih.gov/pubmed/18300086
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2782898/

Here's a Cochrane Review:
http://www.ncbi.nlm.nih.gov/pubmed/21412896

Treatment recommendations are more based on expert consensus than solid research. All the major optometry associations, the NIH eye institute, etc, all recommend office-based VT as the first choice of treatment for convergence insufficiency.

Here's the American Optometric Association Clinical Practice Guidelines. Treatment options for vergence issues start on p. 36.
http://www.aoa.org/documents/optometrists/CPG-18.pdf

Just noticing this one on LD-related vision issues - looks interesting too:
http://www.aoa.org/documents/optometrists/CPG-20.pdf

Also:
http://www.aapos.org/terms/conditions/38

FYI, I get better info searching "convergence insufficiency" than "vision therapy" - but each time I do this, I still seem to find material I hadn't come across before, like those clinical practice guidelines I am now reading...