Thank you for your response. Here are the rest of the details -
Spending rate -
We able to live under $1000 per month because we do not have a house payment.
The money I want to save is for retirement, therefore it is all long term investing. I do not have any big ticket items that I want to buy. IF this is the case, should I be investing 100% of the amount in VTSAX or should I look into other Vanguard funds?
Alright, first let's talk about the cash accumulation...
Typically people hold cash as an emergency fund (e-fund), and it's a good idea to have some to cover the unexpected - job loss, sudden repairs, out-of-pocket medical. How much depends on a bunch of factors including your risk tolerance, how stable your job is and what your monthly 'burn' (spending) rate is. A good place to start is 3-6 months worth of expenses.
With your 'burn' rate that would be $3k-$6k. Adjust accordingly to your own comfort and circumstances.
Everything that is leftover should be invested according to your own personal asset allocation (AA). Your AA is whatever combination of stocks/bonds/real-estate etc. that you feel most comfortable with, and there is no one-size-fits-all AA. Broadly speaking, stocks have outperformed bonds in most decades+ time frames and due to their inherent nature most people expect them to continue to do so.
Around here in the acclimation phase (when you are saving) it's popular to have anywhere between 75% to 100% of your long-term savings in the stock market. Bonds help 'smooth the ride' and can be useful if you get jittery everytime there's a market crash/correction.
Beyond just the stocks/bond question is which sector you want to invest in. For example, VTSAX is the total market index, and a very popular choice because it invests in the entire US stock market. However, because of how its weighted about 71¢ of each dollar invested in the VTSAX is put into the largest US companies - only about 6¢ gets invested in smaller companies. Again, it's fine to have all of your savings in the VTSAX but just understand it only invests in US companies and almost all of it goes towards very large companies (e.g. Apple, IBM, Microsoft, Amazon, Exxon-Mobil...)
If you want more international exposure you might invest some money in Vanguards Total International fund (VGTSX)
If you want more money invested in smaller domestic companies there's Vanguard's Small Growth Index fund (VISGX)
What's important is to decide what your AA should be FIRST, and then invest accordingly. Too many people try to 'chase' the hot market by dumping money into whatever has been going up; that's almost always a method for poor returns.
For example, if you want 100% total US Market, invest everything in VTSAX. Simple!
If you want 60% US Large cap, 20% foreign and 20% US Small cap, invest in VFIAX (60%), VGTSX (20%) and VISGX (20%). Also easy!
Vanguard also has bond funds if you want to incorporate those... Finally, Vanguard has "Target Date REtirement funds" that are designed to do all the balancing for you. They start with a high percentage of stocks/bonds, and always include some international and small-cap exposure. As you get closer to your chosen retirement date the portfolio gradually shifts to include mor ebonds and fewer stocks. A bit more conservative than most here would choose, but it's about as easy a setup as you can get, since Vanguard will do all the rebalancing and adjustments for you. One such fund is VTIVX.
I hope that helps. If you have further questions just ask.