How much citric acid to add to the boil? I don't want overwhelming acidity but I definitely want it to be noticeable
Check out this video, it may help you a little in your goals to get some sour notes into your beer:
As for the Vieille, I'm not certain that it has any bacteria - I did try it once at the brewery, and I can't remember if I tasted any lacto...
I can tell you that St. Bretta DEFINITELY has bacteria in it! It soured an ESB beer when I dropped it into secondary, and did it quick! This beer turned out very nice actually, ended up being like a Flanders red - one of my 1 gallon projects. I pitched the dregs directly from a St. Bretta bottle, with about 1" of beer left in the bottom.
I've also done a starter of the Surette, and pitched it into a Belgian pale ale along with ECY09 and it soured and funked the beer up well, but didn't get it as sour as the St. Bretta.
The post referred to earlier was mine, when I made my first starter from a CS beer - that St. Bretta starter became thick and ropy within a few weeks, and I still have it in a 22 oz bottle with an airlock on it. Last time I tasted was about 2 months ago, and it was thinning up, and tasted like straight pears. I never pitched it into anything, I was too afraid to! It has definitely acted differently than any other culture I've propped up. From what I've read, the thick and ropy 'sickness' is caused by pediococcus bacteria.
The 'guy' at the brewery that I referred to was the guy tending the taps that day I bought the bottle. I mentioned that I intended to try and culture the dregs to make a sour beer, and he did tell me it was an excellent choice, as it was aged in the barrels, and had 'all the bugs'. This was the St. Bretta, and I've confirmed this with 2 bottles by culturing - thar be bugs in them bottles!