The VFINX fund is designed to follow the S&P500 index. This would mean that your position would be 100% in equities. While this is diversified across 504 companies, it would mean that broad movements in the stock market would be reflected within your VFINX position.
You need to ask yourself, what will you do if the market drops 40%?
If you are OK with that, and happy to buy more, then you should be OK in this position. If you aren't OK with that then you'd be better of dividing up that allocation into something with stocks and bonds rather than just bonds. For example, you might want to consider a Target Retirement Fund 2050, which is 90/10 stocks to bonds. You may wish to consider an earlier target date fund that has more bonds it also.
Really depends on how you can handle the loss, because anyone can handle the gain (emotionally at any rate!)
One last consideration is that 'generally speaking' a Mutual Fund (VFINX is a mutual fund seeking to replicate the SP500) is less efficient than an ETF from a tax perspective, so you might want to go the ETF route. Vanguard has VOO and iShares has IVV.