I've solo-leveled 3 warlocks to 70 and I can tell you that as solo playing, affliction is the true zero-downtime build. Especially with dark pact and an imp as a mana battery
As for shadow priests, yes.. you're stuck with them drinking once in a while. With the locks as backup I'd suggest sticking to mindflay/mindblast. Things don't live long enough for VE/VT run their full length making them a waste of mana. I'd suggest using the priest as a tool to keep the warlocks up on health so that they can keep on nuking and just use mindflay to keep the mobs from getting close. The time between pauses for the priest to drink will be significant lower and this less downtime for you.