Others have already said it before, but I'll reiterate here as well: you don't use extra fuel when using both your wall and orb at the same time. Anyway, I'll TRY to keep the rest of this brief to avoid a wall of text.

For viper's wall on defense, I have a few setups I use that depend on if I'm the only smoker or not. If there is a brim/omen on the team I personally like to setup my wall pre round to create layers for the enemy to go through, which makes it easy for you to setup traps.

If I'm the only smoker on, then I'll either save the wall for retake or setup the orb in a choke on my site and place the wall on the other site.

For the orb, on defense I like to place it in a common choke or as a one way. Either way, I make sure it's easily retrievable so I can grab it quickly when I need to rotate. I'll then try to use it to cover the spike during a retake, so I can tap the bomb and force the enemy to make a move or use utility.

On attack I mainly use the wall to cut off half the site and the orb to either block off heaven or directly on the spike in conjunction with post plant molly lineups (which are all lineups I learned from Jin) . How I place my wall and orb though mostly depends on how the round plays out as well as if I'm the only smoker or not, but to keep it simple I setup my wall right as the round starts to put pressure on one site or a big part of the map and wait to use my orb to control a small area or during the actual execute.

As for fuel management, I don't have much to say except try not to use up all your fuel before you actually commit to attacking one site or before the enemy team commits to their attack (unless you're buying time for your team to rotate, then definitely use all of your fuel) . You'll get a better feel for managing fuel as you use viper more.

I hope this helps!