Night and day difference between using EM-1 and ACV. Here's part of the reason why:
EM-1: Lactobacillus bacteria (aka "probiotics" and the ones that create lactic acid), beneficial yeasts and Purple Non Sulfur Bacteria (these are the "magical" bacteria of EM)
ACV: primarily Acetobacter (converts alcohol into acetic acid)
As you can see, ACV has no probiotics. The fermentation process that ACV goes through does result in some beneficial live enzymes which I am not 100% sure but I think EM also creates live enzymes. Anyone making a home-brew EM-like culture...unless they REALLY know what they are doing...is unlikely to have much of the "magical" Purple bacteria. These Purple bacteria are the ones that are primarily responsible for eating and digesting (often into healthful substances like antioxidants) nasty things like chicken poop and other stinky things. They clean up, in harmony with the lactobacillus. The yeasts are there primarily as a food source for the PNSB and LABs (purple non sulfur bac. and lactic acid bac.)
You don't need ACV to make fermented feed. It won't hurt and will add a bit of enzymes to the feed. Mostly it is used to lower the pH of a new batch. It is not really a "starter". A starter, in my book, must contain fermenting organisms, such as LABs. If all you want is to lower the pH of a new batch are not using a starter, ANY KIND of vinegar will work, even the cheapest white vinegar.
As for fermenting feed, EM will ferment the feed much more thoroughly than the plain method (with or without vinegar). I can't prove it (I don't have a lab or care to spend $$$ for lab tests), but go to the EMRO (.org?) site and you can fill your brain with the research that's offered there. Providing a portion of an animal's diet with some fermented food, regardless of the fermentation process, is a no-brainer in my book. EM fermented food will just be "more bang for your buck". The guy I learned from to make EM-FF claimed (he is a scientist, so I assume he tested this) that he would buy the cheapest/crappiest chicken feed he could get his hands on and by the time he was done fermenting it with EM (he used a 4-month process), that it was more nutrient dense than a regular fermented (non-EM) organic chicken feed. I don't care if he is right or not, my "gut feeling" (which has rarely been wrong) about his theory is probably correct. I'd love to see scientific results of this someday.