I tried it a couple times, and the most ilustrative is leaisure walk inside a mall, about 45 mins, with nothing in hands, and watch registered only 0,01 distance.
My VA4 works just fine, I use it when I walk an indoor track and it's pretty darn close. I have tested it outdoors on a known distance just to make sure.
The way I understand it, Garmin does not recommend setting a manual stride length. It is better to let the watch calibrate it automatically by walking outdoors, using GPS. It works fine for me.