I believe that Vanguard has a minimum deposit to open a brokerage account (needed to buy/sell ETFs). I'm not sure what the minimum is though.
An important thing to keep in mind is that you can only buy whole shares of an ETF, not fractional shares. So, if you don't have enough to meet the $3,000 mutual fund minimum, then your shares of VTI aren't going to produce enough dividends to be reinvested, so that money will just sit in your money market fund until you have enough to buy another share of VTI.
I would recommend buying into the 2060 Target Date Fund (or whichever you prefer). Minimum buy-in is $1,000 to start and you can automatically reinvest the dividends and capital gains. When you reach $3,000 or $10,000, you can then think of investing in other funds. With the small amount of money you are currently investing, the difference between the .05 ER of VTI and the .18 ER of a Target Date fund is $1.30/yr (on $1,000).