Do you figure your pace is slower with Vibrams because of the footwear or because of the type of trails you're running?
Or was McCain slowing you down!
McCain always slows people down.
It was the following
a) 23km run the day before
b) 1.5hrs of legs on Sat and a 9km run
c) the heat, and humidy
d) technical trails and vibrams
Looking at your times you seem to be consistently slower with the vibrams... but I don't pay a lot of attention to where you're running.
I find with the vibrams it really depends on how I am feeling....some days I am 15 to 20 secs per km faster with them, other days if I am dragging my feet or tired then I am usually about the same pace, I wouldn't say they are ever making me slower, that is just me.
It is a different running style though, so it wouldn't be surprising if it were different. I'm usually ~10 - 20 seconds faster per km if I'm doing forefoot strike.