Re balancing periodically is a good idea. Some people hold 100% VTSAX only (J. L. Collins’ Simple Path recommendation), with the theory that most large US companies are international which provides some diversification. Others add VTIAX and others to gain more diversification, or any other combination to try to "tilt" towards a certain geographic region, industry, or company size, or add bonds.
I caution about "tilting" because it can lead to timing and emotional trading which rarely works out well. Set an asset allocation and periodic re-balance as needed is about as simple as one can get.
My opinion for most people who want a totally hands off approach is to either buy the vanguard target date index fund for the year they plan on retiring, or to approximate its holdings within your existing accounts. The latter is not quite as simple, but does allow for tax-efficient placement, for example holding bonds in a retirement account (IRA or 401K) and more tax-friendly (VTSAX) in taxable brokerage accounts. Here are the holdings of the 2055 target date fund as of now:
https://personal.vanguard.com/us/funds/snapshot?TTVETF=VETFTEST&FundId=1487&FundIntExt=INT#tab=2Speaking of taxes- this is pretty important to consider in your situation. If you plan on selling VTSAX from a taxable account to re-balance, you will pay taxes when selling. If you are selling shares held less than a year, you will pay even more taxes. If you are in a taxable account, you might be better off either stopping automatic dividend re-investment and instead using them to purchase VTIAX or just directing all new investment to VTIAX until you reach your target.
VTSAX certainly holds everything in QQQ. Buying QQQ would be reducing your diversification, not increasing it. If it is your goal to increase concentration of nasdaq 100 companies, go for it, otherwise just stick with VTSAX.
You can compare holdings here:
https://ycharts.com/companies/QQQ/holdingshttps://ycharts.com/companies/VTI/holdings