Unfortunately you cannot use flexible tube as an extension of a plumbing vent stack and you cannot point vertical vent stack down. It could compromise proper discharge of sewer gases which contain methane (an explosive substance) and push them back into your house.
Based on International Residential Code (IRC) and Universal Plumbing Code (UPC) you could offset your plumbing vent stack beneath the deck surface and come up somewhere on the side (if you have sufficient clearance) but horizontal section must have at least ¼” slope per foot so all the condensation and rain water drains back into the sewer line but the final section must be vertical / pointing up (there are other restrictions that would have to be verified). This offset could be also made before the plumbing vent stack roof penetration, but that would depend on the configuration of your plumbing pipes.
IRC also allows side wall plumbing vent stack terminations (you’d have to check which code is accepted in your jurisdiction) so you could run it under the decking (still requires ¼” slope per foot towards the area where it penetrates roof surface) to some area further away from where you are sitting. However, depending on the wind direction this could still introduce deck occupants to unpleasant odors.
The following side wall plumbing vent termination rules apply (IRC):
It has to be minimum 10 feet from property line
Side wall plumbing vent termination must be minimum 10 feet above highest grade within 10 feet
It may not terminate under vented roof soffit openings
Side wall plumbing vent requires protective screen or other type of bird and rodent protection
The following paragraph is from the 2006 UPC (IRC is pretty much the same):
All open vent pipes that extend through a roof shall be terminated at least 6 inches (termination height would depend on the roof type, slope, and geographical location) above the roof, except that where a roof is to be used for any purpose other than weather protection, the vent extensions shall be run at least 7 feet (2134 mm) above the roof.
The final option would be to extend this plumbing vent at least 7 feet above the decking surface and decorate it so it blends with other roof deck components.
Prohibited use of such installation:
Plumbing vent terminals shall not be used as a flag pole or to support flag poles, television aerials or similar items, except when the piping has been anchored in an approved manner.
Let me know if you have further questions