If you get indecisive, invest in your first choice (S&P 500) and then consider what to do. Otherwise you're deciding on cash by default - don't let perfection prevent picking a reasonable fund that you can change later.
With smaller amounts, you might consider ETFs. Vanguard's ETFs have the exact same contents as the mutual fund equivalents, but they can be purchased in smaller amounts. They trade on the stock market, so for example "VOO" is Vanguard S&P 500 ETF and "VTI" is Vanguard's Total Stock Market ETF. Vanguard allows you to buy it's ETFs for $0/trade.
There's a few main benefits to ETFs. The expense ratio is lower than the investor shares, so you'll keep a bit more of your money. You don't have the minimums, either, because you could buy just 1 share. For example, VTI costs $136/share, so you could invest $136 in the total stock market. Or if you wanted $2500 invested, you would buy 18 shares ($2448) and have $52 left over. Vanguard also has bond funds (BND, Vanguard Total Bond Market) and tax-exempt bond funds (VTEB, Vanguard Tax-Exempt Bond) available to round out your choices. Most ETFs have lower prices, so you probably won't have $52 left over.
Here's an example, let's say you want 10% bonds, 60% US, 30% international with $3000:
US total stock, "VTI", $136.45/share x 13 shares = $1773.85
$3000 - $1773.85 = $1226.15 remaining
Total International, "VXUS", $56.40/share x 16 shares = $902.40 invested
$1226.15 - $902.40 = $323.75 remaining
US Bond Market, "BND", $79.66/share x 4 shares = $318.64 invested
$323.75 - $318.64 = $5.11 in cash left over
Got lucky on that one, with $5 in cash, but you see how the ETF purchases can achieve 60%/30%/10% allocations with only $3000 to invest. Most funds have minimums, and you might only be able to buy one fund. ETFs are probably the only way to grab a 10% bond allocation while meeting the minimums.