Yes, historically speaking (in paper currency environments like today), downturns are corrected in a short period of time. Also, top companies in VTSAX have significant international presence, so you do have some diversification there to begin with.
You seem pretty set already, but the only advice I can add from my own personal experience for a person in a situation like yours is: You are just starting out - not "used" to 70k-90k income yet. It's your best opportunity to develop your saving muscle. If you set 401k, Roth, HSA, and taxable Vanguard investments in 'auto mode' so that you never see the money in your checking account to spend, you'd force yourself to live on the residual and adjust your spending accordingly.
On the other hand, I started out my consuming 90% of my income by frequenting restaurants, online shopping, expensive travel, and then had to scale the expenses down to build savings. Starting out, if you can force yourself to live on 30-40% of your income, you can achieve FI much sooner and will be free to do what you want. Good luck!