Description: A version of vtoclg with an additional disjoint variable condition
(which is removable if we allow use of df-clab , see bj-vtoclg1f ),
which requires fewer axioms (i.e., removes dependency on ax-6 ,
ax-7 , ax-9 , ax-12 , ax-ext , df-clab , df-cleq , df-v ).
(Contributed by BJ, 2-Jul-2022)(Proof modification is discouraged.)