Your 401(k) account became a Traditional IRA when it arrived at Vanguard. If you expect your income will probably be higher in the next several years, you could use a Roth Conversion to turn that Traditional IRA of $3k into a Roth IRA. You owe tax on the conversion, which is why picking a year with low taxes makes sense.
But you could also setup a new Roth IRA with $5,500 or less of after-tax money.
You might also take a look at Vanguard ETFs, since you can buy them 1 share at a time if you like. For example, VTI's (Total US Stock Market) price is $141/share. So you could buy 14 shares x $141 = $1974, and have $1026 left over to diversify into international. VXUS (total international) has a price of $56.89/share, so you could buy 18 shares x $56.89 = $1024.02. So that would leave you 2/3rds in US, 1/3rd international with $2 in cash. ETFs have lower minimums and cost $0/trade, so keep them in mind.