I see similar residue on mine, it's no big deal, the end result of burning 20 gallons of gas is bound to leave something behind, heck no emmisions systenm is perfect I wash mine at least once between fill ups, so I have no idea how bad it can build up over time, but I do see dirt/carbon on my rag when I wipe of the pipe to reveal the brilliant chrome shine microns beneath.

As long as your VX is running smooth, and you are on trak with your maintenance, I wouldn't worry.