Not on purpose, I simply slipped into old habits without realizing it. The reasons I think were twofold. The first is that Vase doesn't employ the left pinky finger until it has to play the A on the first string in the third section, while the arpeggio uses it more extensively. A good hand position is needed if the pinky is to have any real hope of hitting the strings on its tip, while the same is considerably less true of the remaining fingers. I am finding that thinking about the thumb and keeping the hand parallel is less advantageous than thinking about the position of the pinky, as when I do the latter the hand naturally moves to a superior approach. Thinking about it now, this likely accounts for the fact that the third section was less legato than the other sections. Add in the fact that the hour was late plus the fact that I was paying more attention to the music than my hand positions and you saw the consequence. I actually noticed my poor left hand position prior to posting but I just shrugged it off, posted and went to bed. Old habits are hard to break and the parallel position just isn't as natural yet. For what it is worth, it is easier to put my left hand in position now that I started using the guitar support, I just need to keep working at it.
Thanks for the observation. Just remember, do what I say, not what I do! Left hand attack is important and if you share my tendency then you also share my challenge.