I find this counterintuitive to work with, largely because it's not obvious from the display which automation setting you're working with, or even which character you're working on within your title (if you're working on characters individually). 
Could the automation setting that you're working on be highlighted at the left hand side of the display to show that it is active? This simple step would clear a lot of the confusion around use of the PTT.
I've just spent ages adding keyframes to the wrong character of the text. My own fault admittedly, but could have been immediately apparent if Vegas showed which characters and automation settings had focus at that moment.
Jon.