He'd call bullshit if not for the fact vash would probably be a mess when telling him :'| so probably weak attempts at denial followed by asking him how it happened (I imagine the explanation would vary depending on what they remember at that point but the gist of it would be the same), annnnd either try to act like he's okay with how it happened (I imagine this would result in getting yelled at or something by vash because lol) before fucking off, or skip straight to fucking off for a while and get mad at things. Like bears. Get mad at bears and punch them. (He may still do this even if vash follows him, sry vash)

Then go ask around to see if he can go back alive, or otherwise not be dead once his mirror is complete. If he finds out he can, then yay, he'll still have to deal with that but at least he'll know he's not just putting himself back together to die again. If it looks like he can't, then he'd probs cycle through the remaining stages of grief (possibly returning to the anger stage repeatedly) while avoiding putting any more shards in his mirror.

Vice versa for Vash- what if Pound remembers first and then tells Vash about it?