No matter what, you do need an import permit (that's easy to get from APHIS) and a phytosanitary certificate.
Technically speaking,
in-vitro plants are - by definition - artificially propagated, so are exempt from needing CITES docs.
The image below is a screen shot taken directly from the CITES website. The left column is
Appendix I, those orchids that cannot be internationally traded; all other orchids are in
Appendix II. Note the exception comment at the top of the first column.
One might also argue that
in-vitro material is sterile, so exempt from needing a phytosanitary certificate, but some commenters to a proposed rule change affirming that pointed out that if one tissue-cultured a plant infected with a virus, that virus might be carried on, and that leaf-cell nematodes might similarly be transported, so the law did not change.