It seems to be linked to whether I have my weapon raised or lowered, but is not consistent. Sometimes I can running vault whilst my weapon is raised, but when I lower it, my character always does a step-over. But sometimes it is vice-versa, my character will only running vault when I have my weapon lowered. But I have also had times when it works absolutely fine no matter where my weapon is, although I think that is only for a short while after I have logged in and then it reverts to one of the above. I have never had a case where I cannot running vault at all, it always gets stuck on my weapon being raised or lowered and I simply switch my weapon stance to be able to vault again but only with my weapon in that position.
Overall, it is just inconsistent.