Alright, I'm gonna get me some white vinegar and give it a go. I'll report back, would be very interesting if indeed I get better yields with it.

As for checking the pH before or after heating this is what I've found with phosphoric -

When checking the pH after heating I find that for the first boil I don't need any acid as the pH drops naturally to 4 just from having the bark thoroughly mixed with the water (I grind the bark in my blender with water to make a mimosa milkshake, no dust that way).  Once the solution gets hot the pH is 4.

For subsequent boils I need 20-30 drops of phosphoric acid to get the pH down to 4 once it's hot. 

If I get the pH to 4 before heating I need much, much more acid, maybe nearly twice as much and what I found is doing it that way totally kills my yield. Then I'm boiling at what appears to be pH1.

Interested to hear your thoughts.