I've encountered this bug frequently with Vault Scribed Skill, Terrain, PVP, or PVE doesn't matter. I've fought at Mirrormoors, WB's, Dark Anchors, Delves, Group Dungeons, and Cyrodiil, just about everywhere it happens. Even when in an open field I can try to use Vault but it simply won't cast so I've resorted into not using the skill anymore till it's fixed. I tap the button like 3 or more times and it won't cast. I thought at first it was based off the Terrain but no, I've tested it with no edge or hill around and Vault still stalls on casting like there is an invisible cooldown on how many times we can cast it per hour.

I personally like the Vault and I find it useful to get out of AOE's along with dealing DMG to enemies or getting a heal off and debuff your opponents or buff yourself. But because it stalls way too frequently I'd say it's unusable for any and all content in ESO. So much to consider it not worth 50K gold in game, It's a waste to spend gold for a nearly unusable skill.