Vanguard Mutual Funds — Diversification vs Share Class
There's really no right or wrong answer here because you'll be fine either way. If you've investing amounts in the low 5 figures you're likely just getting started, and if your asset allocation is not optimal it's not that big a deal because you have a long time horizon to adjust it, and the expense ratio differences here won't add up to that much. A third option is Vanguard ETFs, which have the expense ratio of Admiral Shares but have lower minimums (i.e. the cost of a single share, typically on the order of $100). However, they are a bit more advanced than mutual funds in that they trade on the market and require you to place orders rather than just specifying the amount you want to buy. A downside here is you might end up with a small amount of cash that you can't invest, since you can initially only buy whole numbers of ETFs shares. So what I'd recommend is buying roughly the correct number of ETFs shares you want except for your largest allocation, then use the rest of your cash on Admiral Shares of that (if possible). For example, let's say you have $15k to invest and you want to be 2/3 U.S. stock, 1/6 international stock, and 1/6 U.S. bond. I would buy as many shares of VXUS (international stock ETF) and BND (U.S. bond ETF) as you can get for $2500 each, then whatever is left over (~$10k) put into VTSAX (U.S. stock Admiral Shares mutual fund).