I would have thought that vault would have more impact. It depends what level you're talking about.
Lower levels: tumbling is harder impact because the vault is landing on your back on a soft mat.
Optionals: I think vault would be harder because you don't tumble all THAT high. Yurchenko vs. full twist? I think the Yurchenko would win on that one.
Elite: Those girls tumble pretty much as high as they vault, three or four times (for three or four passes) vs. just one vault, so I would hazard a guess that tumbling is harder. I say this because there are numerous elite gymnasts who struggle with injuries due to floor/tumbling, but many of the "ancient" gymnasts, like Chusovitina, become vault specialists because it's all their body can handle. Although you'd think there's more impact on vault, it appears that in the long run, this is not the case.
