You are getting 80FPS with Vsync ON? What kind of monitor do you have, best answer would be to tell us your monitor's refresh rate or the model of the monitor.

If the refresh rate is 60Hz which most LCD/LED monitors have then Vsync isn't working correctly. Are you running the game in borderless window mode? Because that can disable Vsync.

If you want to just limit your framerate then the best thing to do is the following: Go to Program Files\Steam\SteamApps\common\dota 2 beta\dota\cfg. In cfg folder, create a new text document and name it autoexec.cfg. The extension must be changed to .cfg. If you can't see extensions then you can enable it in folder options.

Next, open the autoexec.cfg file, it should open in notepad/wordpad. Then type the following line: fps_max "60". This will cap your framerate to 60 frames per second. You can add other values too if you want.