I think the original poster had the best idea. Vinegar, baking soda, and then perhaps cerium oxide. I have tried nearly every product out there that claims to remove hard water stains from glass. NONE of them worked. What I have been doing, because it saves precious time, is with homes I clean regularly, I first spray the glass with a good all purpose cleaner. Using a dry washcloth, I scrub that off, then spray Cinch glass cleaner on it. (Windex with vinegar would work too). It usually takes about 4 cleanings to see real results. Think of iot this way: its great exercise for your arms!!!
If someone approached me about getting rid of it quicker, I think I would go the vinegar, backing soda, etc. method.
Thanks for all the ideas.