The telltales show me different information (or, shows me the information differently) than the wind vane. But the information the telltales show me is a direct result of change in wind direction. Same change in wind direction that turns the wind vane.
The wind vane data will be filtered and averaged to rid of ociliation and "way off data" due to boat movement in choppy sea, water spray etc.
Wind vane placement: Everyone does not place their wind vane in the mast top. Especially not on dinghies. They put it way forward, to be in in line of sight while sailing. Typically on the front of a gennaker pole, where I'm thinking about putting it as well. Look: http://www.murrays.com/mm5/graphics/00000001/08-1295.jpg | http://www.downwindmarine.com/images/P/301893.jpg
However, its hard to make any true conclusions here and now. Hopefully I will be able to test this back in Sweden in late June. Have also got myself a GoPro camera so I might be able to make some footage and we can discuss around that.