A post to the Dota 2 development forum from Valve's Jay Stelly describes their experiments with a new way to get the visual benefits of turning on vsync without the penalty of added latency this normally carries (thanks Gamasutra via NeoGAF). Here's word on how this works, and how it can be enabled in Dota 2:

DOTA 2 includes an experimental feature that allows you to enable vsync with minimal latency. Normally enabling vsync adds another frame of latency causing some inputs to feel delayed. With this new feature the engine will attempt to schedule simulation and rendering to coincide with the start of the new frame on your monitor. It is experimental currently and only available as a console variable. We will add it to the main UI if it is successful for enough users. It requires a PC that can maintain a framerate higher than your refresh rate (usually 60 frames per second or higher).

To try it, enable VSync in the video options panel. Then open the console and set this variable:

r_experimental_lag_limiter 1

You can toggle it on & off to see how it improves your latency.

To turn it back off use the setting:
r_experimental_lag_limiter 0

Please only post feedback about this feature in this thread.