VEXAX is an S&P500 completion index, its purpose is to have all companies not in the S&P500 so that people with only S&P500 inside employer retirement accounts can mimic the total market. Otherwise it is not that special. Vanguard small-cap VSMAX is similar with 0.05% expense, or small-cap-value VSIAX. These would seem to me like better counter weights to the concentration of companies VTSAX, but it may not matter much.
There's nothing wrong with the concept, many people employ it. Sometimes (like so far this year) circumstances favor smaller companies, and sometimes "value" companies. For example the Bill Bernstein "no-brainer" portfolio is 25% to each of VTSAX/similar, VTIAX/similar, VSIAX/similar, VBTLX/similar and it seems like a sound concept to me.