Homemade Buffalo Sauce
Know that the traditional Frank's Buffalo Wings Sauce is just Frank's RedHot and melted butter. I'd definitely start there, and tweak with the substitution. The old standard is 1/2 cup (118ml) Frank's RedHot to 1/3 cup (79ml) melted butter. 

Vinegar is a distinct possibility, to me neither buttermilk nor ketchup make sense.

You might find this recipe for homemade hot sauce helpful. The reviewers love it and it's supposed to taste like Frank's.

EDIT after comments to OP:

Trappey's Red Devil Cayenne Pepper Sauce contains: Distilled Vinegar (Vinegar and Water), Red Cayenne pepper, Salt, Guar Gum, Xanthan Gum, Ascorbic Acid (to preserve freshness).

Frank's RedHot contains: Aged Cayenne red peppers, vinegar, water, salt and garlic powder.

The Scoville rating of Frank's Original is 450, Red Devil is 800-1200 according to these sources: Franks, Trappey's

So, the only significant difference in the ingredients is the garlic in the Franks. The fact that vinegar is listed first in the Trappey's list and second in the Franks list is meaningless, since Franks separates "water and vinegar", Trappey's lists them together.

Frank's claims that the peppers in that sauce are "aged". I looked into that and found that the peppers in some hot sauces (including Frank's) are made into a mash and allowed to ferment before they are made into the sauce. AHA! I suspect that there's your tang.

For the sake of economy and a special "brand" that could potentially set your product apart as superior and unique, I'd experiment with making your own hot sauce starting with fermenting your peppers. Here's a Google search to get you started. I found a lot of those sites informative. Another thing that you can do right off the bat is ferment batches of peppers of varying hotness and mixing that mash with your Trappey's to create mild, medium and hot varieties of your sauce.

And add garlic and butter. 

And ditch the buttermilk :)