As far as I can tell, the MathJax plugin does not seem to work in V3 of vanilla forums. It works fine with V 2.6.4, which is the version I'm still using. The problem seems to involve the rich editor. Is there any possibility that this will be fixed before V 2.6.4 no longer works?
Mathjax should work with Rich Editor. I added the code direct on a test forum and it works too. <script src='https://cdnjs.cloudflare.com/ajax/libs/mathjax/2.7.5/MathJax.js?config=TeX-MML-AM_CHTML' async></script> proof