tubby,
Does anybody know of a simple way of converting vertical speed to an impact g load?
I don't think so really. It depends on a handful of parameters, such as gear design (stroke, spring constant, non-linear behaviour), aircraft weight, whether you're talking a "hard" landing (gear bottoming out), and I'm probably forgetting a couple.
And don't take the 'g' from the FDR or QAR for gospel. It isn't sampled fast enough.