+1 to JL Collins stock series
Since the rise of broad-market index funds there seems to be a fundamental misunderstanding what 'diversification' really is. In previous decades it meant holding 20ish stocks in several different sectors. The thinking was failure of any one company or any particular sector wouldn't wipe you out entirely. Today, holding VTSAX you own something like 7,000 stocks (though its based on market cap, so the largest 500 constitute roughly 80% of your holdings).
That's incredibly diversified as far as stocks go. Your investments will go as the market goes, and you won't risk losing your entire savings unless the modern economy collapses in ways it never has (including the great depression and great recession).
You've also mentioned you have 80% of your assets in VTSAX, and some betterment accounts that aren't 100% stocks. Presumably you are holding some bonds, (or perhaps REITs or cash). Depending on your age and risk tolerance that could be a considerable amount. What about other assets - do you own a house?
If you really wanted to, you could buy a small-cap index fund (e.g. Russel 2,000 index fund) to increase your exposure to small cap stocks (at the expense of larger cap stocks). But that wouldn't necessarily make you any less vulnerable to market downturns - it can often do the opposite, as smaller companies often have a harder time surviving when times are tough.
