You'd have to rewrite the interface for XSteam itself to know what to do with vector inputs; I've looked at that code some in the past and while doable, it would be quite a bit of work given all the functions there are.
It would end up being a (probably recursive) loop to do the multiple cases inside anyway, so probably the way to approach it if you want a higher-level interface that is vectorized is to just write a wrapper routine that takes the input and contains the looping construct to call the calculational routine for the combination of inputs.
