Almost any method you choose to use will suffice to flatten split pipe. I use a hand hammer and the corner of my anvil to get it started and then usually finish up with the power hammer - because that's what I'm used to. On smaller tube, I sometimes get it opened up and then run it through my rolling mill. Whichever way you do it is fine, but be sure to anneal it first, particularly with the harder drawn type L plumbing pipe. Refrigeration tubing (the stuff that comes in coils) is generally sold dead soft so it can be easily formed. The straight type L and type M pipe is, however, sent out at half hard or full hard and if you don't anneal it is both harder to open up and likely to crack when being flattened.

BTW, welcome to ArtMetal!

Joe,

Acetic acid for photographic use (stop bath) is, as I recall, pretty high-strength. In my darkroom I used glacial acetic (99.9%), though I don't remember the final dilution for the stop bath. I do remember that the glacial acetic would give you a really deep burn in no time flat if you splashed it on your skin. Ouch - still have a couple scars from that.

Since vinegar is usually around 6% acetic, I'd probably use about that same dilution or just a bit stronger if I was to make pickle using glacial acetic.

I'm sure you're already well aware but I'll repeat here for those who may not be: Always add the acid to the water, never add water to acid. If you put water into acid, it will create an exothermic reaction that may well cause a steam explosion.