How hard can it be to make vinegar?

[Grasshopper]

I have been brewing ales from extract for a couple of years and last winter made my first attempt at a lager. I attempted a high gravity recipe and relied on the cool temperature in my garage to keep the ferment temperature low. This effort didn't turn out well; it took forever to completely ferment as the yeast went dormant during a cold snap. I brought it into the basement and finally got to the target finishing gravity and estimated ABV of 9% after 3 months. Unfortunately the beer came out a bit too sweet and off balance and I haven't found anybody that really likes it so I thought I would make some malt vinegar out of it. I diluted it down to about 7% alcohol and added white wine mother to it in a carboy with cheese cloth over the top. After a month there was little vinegar taste so I pulled air through the liquid using my all-in-one pump. There was still little vinegar taste after 45 days so I repeated this process. Now at 65 days there is still no change but fortunately no off taste either. I just added 6 oz of Bragg apple cider vinegar with mother to it and will see what happens.

Should making vinegar be this hard? I don't seem to have any problem making a half opened bottle of wine in the fridge taste of vinegar after a day or two. Was the original mother inactive? Do hops inhibit vinegar formation? Any thoughts?