Thats great information, I hadn't looked closely at the perks or considered dual casting. If I don't have to worry about speed-leveling illusion, then I can probably pay more attention to things like lockpicking and speech.
No problem. You still might want to level illusion fast though. You'll want the +level perks fairly fast otherwise you'll have to dual cast everything. At 75 you'll get invisibility and at 90 you can use the illusion effects on undead, daedra and automatons as well. Luckily illusion is one of the easiest skills to level.
By the way:
- Vampire +25% illusion bonus works on level cap, including perks.
- Master spells can't be dual cast. So in the end there might be some enemies that are higher level (but not for your dual cast expert spells). Drinking an illusion potion should solve this when you encounter these rare situations. Potion bonus on the other hand only affect the spell level, not perks.
What enemies are immune???
Haven't yet met any normal types besides "undead, daedra and automatons" before that perk. Undead especially are abundant and therefore a bit tricky sometimes. With restoration you could make them flee and then stab 'em in the back in the meanwhile.
Dragons are definitely immune to illusion though and I wouldn't be surprised if a select few more are immune to magic in general or something. For 99.8% of your encounters though, a fully perked illusion tree = le awesome.
And are you going armor-free? How do you handle it when you get noticed or otherwise screw up?
Read the http://www.gamesas.com/index.php?/topic/1312052-master-playthrough-40-hp-vampire-assassin/ I posted

I'm playing on master difficulty and not putting any points in health so whenever I screw up, ever so slightly, I die. Makes the game a lot of fun I have to say. I'm afraid of everything, yet extremely capable of beating everything as well. Good balance that way.
Having said that, I do level up light armor for the perks. The 50% stamina regeneration one is nice (especially during daylight as a vampire since otherwise you don't regenerate *at all*) and a 10% dodge isn't too bad either. Currently I am using some thieves and brotherhood mix and match gear for the +sneak enchants though, till I enchant my own armors.
I refuse to wear heavy armor as I don't want to level up any skills I don't use anyway (nor read their books). It will only make the enemies stronger but not me.
So in short, I focus on:
- Illusion
- Sneak
- One Handed (Back'smashing' with maces with 12x back'stab' bonus)
- Light armor
After that some points in Restoration to get the mana regeneration perks (and stamina when healing). Smithing and Enchanting is optional.
Paralyze in the Alteration tree is cool as well, though I might not invest any perk points in it. Same with Conjuration. Lategame I perhaps will have enough perk points to make them permanent and/or have two of them at the same time but that's not needed for now. Besides as a stage 4 vampire you already have a summon "powerful dead body" spell once a day.