What's up Jonathan! You said it right, the mild acid vinegar test simply tells you if the rock is soft and chalky, and will dissolve in your tank, like limestone... FWIW, you might WANT limestone in your Rift Lake aquarium. I know I do! So if it bubbles, I am happy!
"Rocks from outside" can mean many, many different things... I hope I don't have to post ANOTHER picture of my 75 that is crammed to the brim with huge limestone "rocks from outside" and has been for two years... the billion fry in my tank must have adapted to all that metal, because they are doing wonderfully, lol...
I've always, always used bleach to disinfect the rock, and rinsed in a Prime wash. Never lost a fish, not once. This has been posted about a skillion times. People are so afraid of "rocks from outside" that they are willing to pay a hefty sum for rock from their LFS to the tune of many many $$$ per pound. I guess those rocks grew inside somewhere???