This is how I leveled all of my priests.
Wand spec 5
Spirit tap 5
SWP 2
Shadow Focus 3
Respec
Spirit tap 5
SWP 2
Shadow Focus 3
Mindflay 1
Imp Mindblast 4
Shadow weaving 5
VE 1
IMP VE 2
Focused mind 2
Darkness 5
Shadowform 1
Shadow power 5
Misery 4
Vamp Touch 1
Wand spec 5
Improved shield 3
Improved Fort 2
Inner Focus 1
Meditation 3
Martyrdom 1
Mental Agility 5

This build is to decrease downtime and cost of all spells resulting in less down time.
Your spell order after VT should be..
VT
SWP
Mindflay
Mindflay
Mindflay
Wand

Sometimes you can swap out the wand for a mindblast if your gear has enough spirit on it or you have alot of mage waters. This was an effective build for me PVE leveling as well as for PVP while leveling.
I specced holy untill I was high enough for shadow form then I went shadow but I also leveled as shadow priest this way (before vt) For levelign I just did instances and quests really, until I got to 53ish and me and a mage friend aoe grinded lumberjacks. If you can find a mage (Ice preferably) that plays often at the same times as you then your best bet would be to healbot them.