Dealing with mana issues - Shadow

I have a question about the effects of both Vampiric Touch and Glyph of Shadow Word: Pain spells. With both you gain 1% of Maximum mana per 10 sec. The only difference is triggering. VT triggers this effect after successful Mind Blast and Glyph of SW:P triggers on ever tick of SW:P.

What happens if you cast both VT and glyphed SW:P? Which effect is apllied? I am pretty sure it is not both, but then...what is the use of that glyph? For those who don't use VT + MB combo?

Another thing, I've heard that following two talents cannot stack with each other:
Shadow tree : Improved Spirit Tap (33% mana while casting - for 8 secs, after crit)
Discipline tree: Meditation (50% mana while casting - permanently)

What happens here? If crit happens, mana gain while casting is 83%? Did someone checked if those two are stackable?
Working WoTLK calc : http://wotlk.openwow.com/talent#b
My idea of full tree: http://wotlk.openwow.com/talent#bVcbhZZGxfVofzffqfzAo
I am asking all this because since I leved and played Shadow Priest in endgame, I always had mana issues, so this time at Lordaeron - I want to fix that with talents...

Thank you for your answers and opinions!

If you had mana issues at end game as a Shadow Priest then you were doing something wrong.

Between Shadowfiend, Dispersion and Replenishment you should never go OOM.

You should not be using the Glyph of Shadow Word: Pain.

Yes the two talents stack. Even if they did not stack, it is worth it for the 10% spirit buff which gives us additional spellpower and mana regen.

Ok, maybe I didn't descibed as much as possible, lets say that I had mana issues in fights with more adds (a lots of Mind Searing), when there is one boss and few adds, there were no problems. We will se what will happen when I reach the endgame here on Lordaeron...

Yes, I used Shadowfiend and Dispersion as mana refillers, sometimes at the same time for full mana refill. I never used Glyph of SW:P exactly because the question I asked.

Others (many of "I am the best DPS, you should do as I say") might say that Improved Spirit Tap is useless, but I disagree, it is very very useful for both Spellpower and mana regen, so i agree with your theory, kalvindeane. Thanks!

As kalvindeane said, I don't think shadow priests should have that much mana issues tbh. If mana is a problem for you, I would go for something like this: http://wotlk.openwow.com/talent#bVcbhZZGxfmRfzffqfzio Taking a point off of pain and suffering, since you don't need the 100% chance on the refresh since you will be using mind flay a lot anyway. If you want to have 5 points in improved mindblast, take a point out of focused mind. I think Veiled Shadows is a really good talent which I, at least personally, like to include in almost every build that I use when playing shadow.

And to add to that, I used to have some mana problems, until I learned how and when to use my mana abilities, (ie. shadow fiend at like 50% mana etc.)

Thanks for commenting Touchooo.

IMO, Improved Mind Blast is pure DPS gain before having BIS armor, so I would'nt remove any points from here. I partially agree with Pain and Suffering 2/3 because it can happen that Mind Flay doesn't trigger in the time of SW:P. If that ever happens -> DPS drop. But I will surely test that later on.

I was applying Veiled Shadows before, very useful talent, I will try to play with points to put at least 1 point here, tough decisions....