Generally you do a priority based system, not a strict rotation. Dots no longer clip (so you don't have to wait for the last tick to fall off before refreshing) This means you can just refresh dots when the duration starts to get low with no loss to dps. If you are level 80 with semi decent gear you can go full bore (VT -> DP > SW
etc etc) on bosses without any mana issues. If you are leveling/and or fighting a lot of trash mobs they may die too quickly for a full rotation of your dots to matter or be mana efficient. However, this lets you do cheese like VT + DP + Pain one mob, VT + pain another, mind flay here and there, and wear down groups of mobs at a time, then disperse or SF to get back up and repeat.
Shadow priests are extremely mana efficient overall, and it mostly comes down to just prioritizing your dots, nuking, and then mind flaying in between. Wanding is unnecessary at 80 now, but I can see its place if you are still leveling pre 80. Not really sure what your situation is though, so I'm just trying to provide general advice.