Over the years I have come to the conclusion that December is often a really great time to selectively buy certain stocks, especially when the year went mostly in a single direction - either up or down.
2016 was a generally pretty good year for stocks and a decent year for VTSI. So, maybe the seller is selling some in order to offset losses elsewhere? Usually we all tend to think about weakness in a stock being due to tax loss selling but sometimes it doesn't work in the obvious fashion even though the same technical are in play.
It would be more clear if the seller would simply post his/her reason on this message board.
Bigger question for me is why no one has stepped up and bought those shares. Sort of a rare opportunity to nab .15% of the company without moving the stock!