From a completely technical point of view, it depends on a lot of factors. Size of the pole vent, volume under the DC, excursion etc... You can go crazy figuring out the variables.
Or you can just say that at least 3/4 of the diameter of the pole vent to the nearest barrier. More is better, less may not hurt too bad, but really close is likely going to have a small impact. Blocking the vent significantly will likely cause issues.
I have done systems (mainly IW subs) where the pole vent is very close to the back wall of the cabinet, but the amount of displacement and air movement in the vent area is small and it works great. If you have barrier issues with a brace that is somewhat narrow and has a lot of open space around it, don't sweat this too much. Common sense is cool.