I see this question come up constantly so I'll give my honest answer after 8 years of investing.

VTI covers the entire US stock market — large caps, mid caps, small caps. The expense ratio is 0.03%. For most people building wealth over long time horizons, yes, VTI alone gets you enormous diversification at basically zero cost.

That said, I personally add VXUS for international exposure because the US is about 60% of global market cap and I'd rather hold something closer to the global total market. Whether that extra diversification matters in practice is debatable, but it doesn't cost me much.

The honest answer: VTI alone is fine. VTI + VXUS is also fine. The gap between those two portfolios is smaller than the gap between either and doing nothing.