It's probably a good idea to look at what Vanguard does in their all-in-one funds. Their default advice for equities:
60% US Total Stock Market
40% Total International Stock Market
You don't have direct access to those funds, but you can approximate them by combining the funds you have.
Roughly 80% VFIAX + 20% VSMAX will approximate the US Stock Market.
Roughly 80% VTGMX + 20% VEMAX will approximate Total International.
So,
if you wanted 90/10 stocks/bonds and 60/40 US/International, here's a reasonable allocation:
45% VFIAX
9% VSMAX
29% VTGMX
7% VEMAX
10% VBILX
You should check my math, though.
As others have said, there's no optimal portfolio a priori. Instead, there's a fairly broad spectrum of rational portfolios.
More information can be found here:
http://www.bogleheads.org/wiki/Approximating_total_stock_markethttp://www.bogleheads.org/wiki/Domestic/InternationalEdit: I just wanted to point out that as Jack Bogle says "Successful investing involves doing just a few things right and avoiding serious mistakes". So, don't get
too hung up on the details