I noticed your post from a few days previous but I do not have a complete answer. There are different codes in different parts of the country and this is governed by the Mechanical Code of your jurisdiction. My Uniform Plumbing Code answer may not be applicable to you.

However, plumbing vents are 10 feet away from windows and vent stacks, etc. Gas Water heater vents are supposed to be 8 feet from an obstruction. I don't know if a gas vent is considered a vent stack for the 10 foot rule or if the plumbing vent is considered an obstruction for the 8 foot rule.

I'll ask but I don't now know the distance apart for a plumbing and gas vent.

I always thought that the height of a gas vent must be above the highest point of the roof. I can't find any cite in the mechanical or plumbing codes.

The 2003 UPC 510.6.2.1 "...All other gas vents shall terminate not less than 2 ft above the highest point where they pass through the roof and at least 2 ft higher than any portion of a building within 10 ft."

thanks for the info....