Ofer, I don't think it will be possible to get rid of vibrations with those huge focal lenght you're talking about, in case you're not cementing it to a rock!
I've been using 300- and 500mm for the Canon XL-series for some years and even with a monster of a tripod system I get vibrations in some grade in my footage.
To minimum the vibrations I try not to touch the set up while recording, or when I have to do a pan, adjusting the drag of the head to very high! I this cases you got it as good as it can be.
Link to my tripod setup:
http://www.dvinfo.net/conf/attachmen...0&d=1196152047