In the United States, the conventional wisdom has been - and continues to be - that malt vinegar is NOT SUITABLE for someone on a gluten-free diet. This rationale is based primarily on two important factors: 1) That malt vinegar is made from barley, a gluten-containing ingredient, and 2) That, unlike other vinegars, malt vinegar is not a distilled vinegar. (For more information on distillation and gluten-free foods, see this post.)
However, recent research suggests that this degree of caution may be unfounded. I won't go into the finer points of the science here, but the main gist is that the process by which malt vinegar is made systematically and incrementally breaks down its gluten (and/or lowers its concentration) to sufficiently low levels to make it acceptable for people on a gluten-free diet. For instance, the process of malting breaks down some of barley's hordeins (the gluten protein in barley). The mashing part of the brewing process as well as the yeast fermentation both continue that sequence of further breaking down barley's hordeins. Lastly, the malt vinegar is then diluted with water (typically to 5% acidity). (It's also worth noting that malt vinegar is almost always used as a condiment in low quantities...) Again, the idea is that at each stage of the malt vinegar production process, the gluten is either a) broken down, or b) diluted.
The end result is that malt vinegar in theory contains very little - or possibly even no - in tact gluten. Surely, this will come as a surprise to many in the United States. It may even be considered controversial, or dismissed outright. But consider that Coeliac UK, the leading non-profit focused on Celiac Disease in the United Kingdom, maintains that malt vinegar is ACCEPTABLE for people on a gluten-free diet. (See the 3rd question under the gluten-free diet FAQ here.) This is major news for us US-based gluten-free foodies.
It goes without saying that you should consult your doctor before you consider adding malt vinegar to your gluten-free diet. I haven't exactly gone out and started chugging half bottles of malt vinegar to test things out. But I did recently attend a dinner at a friend's house, and they served jerk pork. The recipe included 2 tbsp malt vinegar to every 2.5 pounds of pork, and when all was said and done, I didn't get sick. I'm only a sample size of one, and hardly representative of all gluten-free foodies out there - we're a diverse group with different conditions (Celiac, gluten intolerance, wheat allergy) and different levels of sensitivity. But I do believe we're living at a time when some of the conventional wisdom surrounding the gluten-free diet may be overthrown, if not revised.
Only time will tell what reforms take hold...