Using Vsync will decrease the amount of frames your card has to render, which would directly decrease the load and hence temperature of the card as well.
Hence if your card is rendering at over 100FPS, it'd be running warmer since it renders more frames than otherwise needed. Vsync would be especially recommended for older games since you really don't want to be stressing your card running them at 200FPS.
Vsync does cause mouse lag in games, but it eliminates unnecessary load off your GPU so that your framerate matches your monitor's refresh rate.