Thanks guys.
The B777 FCTM states that the use of a 2 bar VASI system is NOT recommended, as it provides a visual aiming point that results in the main landing gear touchdown at, or very near, the end of the runway threshold. The main gear height over the threshold with this system is 19' for the B777-300ER, and 20' and 22' for the B772 and LR respectively. Threshold to main gear touchdown with no flare is about 365' - not a huge safety margin.
The use on the other hand of the 3 bar VASI system, results in in an increased safety margin in respect to the threshold height, but may result in landing further down the runway. The main gear height over the threshold is about 70'. Although this is only 20' above the normal 50' TCH, it makes a large difference on the touchdown point. The threshold to main gear touchdown point with no flare in this instance is 1200'.
I presume that when using the 3 bar VASI, wind in another notch of autobrake - just in case!!!