To answer your question yes this would be an appropriate thing to do following an emergency descent with the oxygen selected to 100%. At the level off altitude, provided it is reasonably low i.e. 10,000-15,000ft, then oxygen should be selected to normal in order to preserve the crew oxygen supply for as long as possible.
On the subject of harmful vapours and how to set up the oxygen mask my Boeing books recommend that you select 100% oxygen along with the emergency setting, which supplies 100% oxygen under pressure. Although admittedly at the 100% setting you are receiving exactly that, 100% oxygen and no cabin air, the positive flow setting provides an extra levlel of protection from any nasty vapours entering the mask.