Isn't it better to go with VTI until you have $10k to move into VTSAX?
The difference in expense ratios between the two funds is 0.11%. Investing $5,500 in VTSMX for a year will cost you about $6 more in fund expenses than investing in VTI. On the other hand, investing in VTI you'll lose a few cents to the bid/ask spread on both the buy and sell side, plus you'll have a few bucks of cash sitting on the sidelines not invested because you'll need to buy whole shares of VTI.
Sounds like basically a wash to me.