1) Have fun searching for the "optimum," but be aware that you'll mostly be kidding yourself. Like the quest for the optimum beer, there is endless argument and nobody really knows. The people who sound most confident probably know the least--or are selling something.
2) Beware of soundbite arguments--short, snappy, convincing-sounding slogans.
3) There isn't any objective optimum. The optimum for you depends on your personal level of risk tolerance. Task number one is to determine that in the best way you can. Don't let other people tell you what it should be, say that because you're young it must automatically be high, etc. Selling off in 2008-2009 did more damage to peoples' portfolios than any subtleties of weighting.
4) The portfolio you mention is not "heavily weighted in favor of US large cap/core/growth." Both Vanguard Total Stock Market and Vanguard Total International include large, medium, and small-cap stocks. They are "capitalization-weighted," meaning weighted equal weight voting by dollars. Most index funds use cap-weighting for a number of reasons. It is a theoretical optimum under some theories, it offers cost advantages, and is widely regarded as being fully diversified.
Of course, there are many products (and advisory services) competing with index funds. In addition to active management, there are an increasing number that use passive techniques but use some modified or "tilted" weighting. A skeptic like me would say that they have to do something different from an ordinary index fund or they wouldn't have a reason to exist--and that the firms selling them have to publicize any plausible reason for believing these weighting systems are superior.
5) Beware of the soundbite argument that cap-weighted indexes are "concentrated" in large-caps. Well, it's not crazy that some other weighting might be marginally superior. But it's far from cut-and-dried--but claiming that equal weighting by dollars is "concentrated" seems like spin or sloganeering. To me, it's like claiming that a bread recipe calling for 3-1/2 cups of flour, and 2 teaspoons of salt, 1 teaspoon of yeast is "concentrated" in flour and "underexposed" to salt and yeast--simply because the amounts are unequal.
If you want to go with a Schwab "fundamental index" fund instead of Vanguard, or perhaps use the TILT ETF, nobody will say you nay. But do it because you feel these departures from cap-weighting are better than cap-weighting, not that cap-weighting is intrinsically wrong just because it holds more Exxon than Millepore.
6) The portfolio you mention, with equal quantities of U. S. and international does not overweight the US. is in fact very close to world stock market capitalization. If you think the difference can possibly matter, it is certainly easy enough to changes the proportions to make the stock allocation be 46% US and 54% international, i.e. 30.7% Total Stock, 36% Total International, 33.3% Total Bond. But I don't think even the most vigorous advocates of international investing in this forum would claim that that changes matters much.
7) If you want a simple portfolio that incorporates the thinking of the "slice-and-dice," multi-asset, value and small-cap-value tilt faction, look into Bill Schultheis's "Coffeehouse Portfolio."
8) Finding some optimum allocation within bonds is less important than within stocks, because the entire universe of bonds isn't as diversified as the entire universe of stocks, and because bonds fluctuate so much less that if you have any significant stock allocation, the behavior of your portfolio is going to be dominated by the behavior of the stocks.
9) Finding some optimum allocation within either bonds or stocks is less important than the simple, basic decision of what percentage of your portfolio should be in stocks. The biggest question about the 1/3-each portfolio is not the stocks and bonds in it, but the basic proportions of 2/3 stocks, 1/3 bonds. Not that that's unreasonable, but it's a cookie-cutter one-size-fits-all proportion.
Annual income twenty pounds, annual expenditure nineteen nineteen and six, result happiness; Annual income twenty pounds, annual expenditure twenty pounds ought and six, result misery.