GW - I am an Engineer and a Geologists and do soils Engineers for a living. In addition, I build ponds on the weekends so I have a little practical experince.
1. As far as compaction goes, sheetsfoot rollers are what are used to proper compact clay.
Typically vibratory rollers are used on granular soils. When you design a dam you usually would like to get the clay core compacted to 90% of maximum dry density. You can get this from a standard dozer tread but it is easier to get it with a sheepsfoot. Just as important is the moisture content of the clay, typically you want it on the wet side of optimum so if there is a little settlement it will be selfsealing.
I just finished building a one acre pond this weekend and relied only on my dozer for compaction. In my area this works well so if your contractor has had good results, try it. I think proper moisture content is probably more important than % compaction.
