From my own players performance and scouting many players on good teams I've found nothing conclusive about positioning, vision or footwork with the exception that vision helps at QB. Their effect seems to be far outweighed by that of primary skills, BPOS, chemistry and energy.
And I've seen no evidence that positioning rather than tackling should be the primary skill for DBs.

Well you have a good point but with this game engine the majority of TDs are long passes!
Hence, to do well, stop your opponent scoring and win games, tackling is more important than positioning for DBs. But I'm not saying to ignore positioning.
After the Iron Bowl last season I scouted the DBs of teams involved and I've been steadily working my way down through top teams since.
Scout the Iron Bowl winner's DBs and you will see for yourself.

positioning can be coached vision can not.BUT vision can be corrected, case in point. Bob greise of the miami dolphins back in the 1970"s was the first to weare sports prescription glasses. Just my 2 cents worth hope it helps.[teamid=]