While both funds are excellent funds, they really are not well diversified like the VTI. These may not be good for a traditional diversified portfolio.There are single funds that you may find acceptable. Wellesley and Wellington are not index funds but they have long records of consistency. Wellesley is 1:2 equities:bonds, and Wellington is 2:1. The more equities, the more it will drop when 'the market' drops, but the more it will grow over time. Expect drops of value as long as 5 to 7 years, but the dividends keep coming.
When I read up on diversification years ago I caught one tit bit. Adding a diversifier like a Goldman Sachs Commodity Index (GSCI) to a equity/bond diversified can reduce risk. The example they used was 2% GSCI. GSCI had a high standard deviation, but reduced the standard deviation of the overall portfolio. There are modifications like this that can be used to enhance the diversification unless you believe this is covered in VXUS.