If GetAsyncKeyState is issued twice in succession while the key is down, the second GetAsyncKeyState reports the key as being up.
Virtual keys for regular characters like VK_A, VK_B, and so on are not supported -- just as they are not supported for GetKeyState.
For mouse Vkeys like VK_LBUTTON, the synchronous state, rather than the asynchronous state, is provided becasue PM does not provide any way to get the asynchronous state of the mouse buttons.