If we had the Vanguard Total Stock Market Index Fund in our 401k, I'd be 100% in it. But yes, by all means, make sure you check the fund's fees in the actual 401k.
I only have maybe 2-3 Vanguard options in my 401k, so I ended up 100% in VIGAX instead. It's done well enough. I looked at expense ratios and 10-year performance for the various options, and the non-Vanguard ones were much worse than the Vanguard ones. You're lucky that you have a Total Stock Market Index Fund option; a lot of 401k's don't have any good low-cost index-fund options.
Bear in mind that your 401k does not have to be your only investment, and that VTSMX/VTSAX has diversification built right into it, as would any other total stock market index fund.