Just how do I do it more consistently

Just know it works way more consistently in the demo than it does in the full game. The devs patched out the clip in later versions of full game, so it's probably just better to get the old version.

If you're on demo though, just spamming E will work. for full game just crouch into the door and jumping WHILE holding forward (this should trick the game into doing a vault through the door).