I haven't looked too much into vaulting technique, but one of the vids I saw for speedvaults said you should launch with the leg that's on the same side as the arm that you touch the wall with. I've tried practicing this and can do it okay if I concentrate, but often I still freak out at the last moment and use my other leg as it seems more natural to me lol.
Watched the start of "Jump Britain" last night (amazing - not so much the stuff around the landmarks but just the stuff they do in the spots around town, it's got me really looking forward to Saturday

) and realised that everyone kind of does things differently anyway so maybe it doesn't matter which leg I use? Or is using the same-side leg necessary to get the fastest vault? I should probably practice with both anyway, and practice vaulting using the other arm to get better control of my body.