In Three.js v151 there were changes that removed uv2_vertex and uv2_pars_vertex which cause issues loading VRM files that have the mtoon shader.
Here is the Three.js change: mrdoob/three.js@5c5a5c7#diff-c2bc4656a48180f106fc5f8813c5d6fd1fe51f8af74b7ac5e1939661379d4841
I found that mtoon uses uv2_vertex and uv2_pars_vertex in Line 42 and 17 of the mtoon.vert file.
I attempted to comment them out, but as expected it broke things further. Reporting here as I try to solve the problem. :)