My best advice would be to simply play. Try out different things, see what things seem more difficult... For example, with improved Mind Blast, I almost never use Smite or Holy Fire, and this is likely to be even more so now that I have Shadow Weaving (which decreases the target's shadow resistance). I tend to stick to shadow spells only, now, but without those talents, I'd probably use Smite and Holy Fire more often (especially since the latter is a DoT).
My rotation isn't based on any kind of math, so I don't know whether it's "the best", but it's what I do:
Mind Blast, Shadow Word: Pain, Shield, and if they're melee, Touch of Weakness. Mind Blast again. Then, if still at least half health after all that, Mind Flay until 1/4 health, and wand after that. If my mana is doing well, sometimes I'll throw in a third Mind Blast after a Mind Flay, but that's pretty rare.
I'm an Alchemist, so I always have plenty of mana potions. If I'm jumped by multiple mobs that I can't really handle easily, I take a mana potion. And I'm always forgetting to heal myself, unless I'm on the brink of death. I used to carry around healing potions, but I never used them. These are simply matters of style. So, you'll have to figure out your own style. I don't particularly like using the wand, since while it's rotating, I can't do anything else, and I'm not very adept at turning it off. So, I only use it at the very end of a fight or when my mana is really low. Other people use wands a lot more.
Another idea might be to start leveling a Shadow priest alt, so you can see what it's like at the lower levels. A lot of what we take for granted with our class/spec is stuff we learned earlier and just do by rote after all that experience. I'm currently leveling a Holy/Disc priest on another server and sorely miss my Spirit Tap (but this is going to be mostly a grouping character, so it will be moot once I start grouping more). Once I'm past Wand Specialization and Healing Focus, I'll probably start noticing bigger changes and adjust my playstyle accordingly.
Edit: I'm level 38. I can't address all of your issues. Also, I'm not 100% Shadow. I have a fair number of points in Discipline (I want to eventually get to Meditation) and 2 points in Holy (uninterrupted healing). So I don't have Vampiric anything yet.
Edited, Feb 7th 2008 2:24am by Vespassienne