Hello.
I could be wrong, but there is no setting for any of the targets (ipods, sansas, for vibe there is no for sure). I've set it so sensitive because it is required with longer lists IMO. It's ok for me. However it would be nice to have control over that in the future. Anyway, right now there is one solution: you could try to compile your own build and replace one line in firmware/target/arm/pbell/vibe500/button-vibe500.c from: "int scr_pos = val >> 8;" to "int scr_pos = val >> 9;".
You should read
http://www.rockbox.org/wiki/SimpleGuideToCompiling, if you don't know how to compile the source.
EDIT: I don't know... I turned on my vibe to remind myself how it works (because i use the ipod video everyday) and it's really not so bad IMO. I can reach every position without a problem, when i move my finger slowly.