So one thing you'll need to know is that no, you're not too heavily invested in index funds, even though you are 100% invested in index funds.
The Target 2050 Fund itself holds three Vanguard index funds:
1 Vanguard Total Stock Market Index Fund Investor Shares 62.9%
2 Vanguard Total International Stock Index Fund Investor Shares 27.2%
3 Vanguard Total Bond Market II Index Fund Investor Shares 9.9%
https://personal.vanguard.com/us/funds/snapshot?FundId=0699&FundIntExt=INTThose three funds comprise the entirety of your 2050 fund. By holding VTSAX outside the 2050 fund, you are tilting your holdings towards this fund, so the returns of VTSAX are going to have an outsize impact on your returns vs if you only held 2050.
In reality you cannot be "too heavily invested in Index funds" because Index Funds are not an asset class in the way that stocks, bonds or REITs are. The question you should be asking is "Am I holding the right mix of index funds for me?" So what are your goals for this money, what sort of returns do you expect, and what sort of risk can you tolerate? Is 2050 a realistic retirement date? (definitely not if you're shooting for FI!). If the market were down and you lost 30%, 40% or even 50% of your money would you panic and sell, or hold for the long haul? The answers to these questions will tell you what asset classes to hold.