Elaine, I gather that you have both taxable investments (VTSMX), a 401(k), and now you're thinking of an IRA. All good decisions. You really need to look at all of your investments as a
whole portfolio and not as isolated accounts. No one can really advise you how to invest in an IRA without also considering what you're investing in the 401(k) and what other investments you have.
Generally, you'll want to start with a broad asset allocation -- what percentage of your portfolio to hold in stocks, and what percentage to hold in fixed income (bonds, cash). That's a very personal choice. A reasonable starting point might be 80% stock and 20% fixed income, although others might suggest doing more or less stock than that.
Next, of your stock investments, what percent do you want to hold in US versus international stocks? A reasonable default holding would be 70% US and 30% international, although some argue for less international while others will suggest holding US and International stock at their respective market capitalization which is around 45% US and 55% non-US.
You can then break it down even further if desired into even smaller asset classes.
Where does this leave you? If you are already investing in VTSMX (Vanguard Total Stock Market Index Fund), then you ought to have some complementary holdings in international stocks and bonds. And we don't know what you have in your 401(k).
One thought is to leave your VTSMX where it is, and then just select "target retirement date" funds in your 401(k) (assuming it has good options there) and in the IRA. With target date funds, you would be assured to have a good investment balance. You should pick the target date funds that come closest to your desired asset allocation (stocks vs bonds) but lean a bit more on the bond side given that you have 100% stock in VTSMX which pushes your overall portfolio more into stocks. You could certainly do worse than this strategy.
I'm not trying to make this sound difficult, but you should try to ensure that your overall portfolio is well thought out and not just a grab-bag of different funds that don't necessarily make sense together.
BTW, a really good introductory book on all of this is just 99 pages and $5 (on Kindle/iPad) here:
Investing Made Simple: Index Fund Investing and ETF Investing Explained in 100 Pages or Less.