I’ve gotten my main’s and my main alt relatively easy and it has seemed that it’s 100% drop (From Torghast and raid or world boss).
But I’ve heard the more repeatable the content is, the lower the drop rate, checked wowhead for the memory and it’s a stupidly low 0.3% (Which would be beyond dumb if that’s true). Is that right? It’s kinda the recommended legendary for dungeons, but at any rate I’d just use Memory of vampiric aura and not waste my time supporting ridiculously low drop rates. 
That low drop rate is across all classes. As your memories drop only for your class, the numbers are skewed.
Additionally, they’re no longer on the loot table for people who have already found it, further decreasing the percentage on Wowhead.
That being said, some people had to run heroic 50, 60 times before they saw what they wanted.
You can buy a random memory for 35 Grateful Offerings. It never gave me a double and I bought most of them using those.
Oh my.
I need to have 4 anima conductor locations empowered, right?
For me between my Mage and this Druid, I’ve been able to get the majority of my Memories with relatively few attempts.
However, I will note that any I didn’t get when doing Heroic or M+0 usually landed after 1-2 attempts in medium M+ keys.
There’s been one exception, and that was getting The Dark Titans Gift for Resto Druid off the last boss of Halls of Atonement. I think that actually took me nearly a dozen attempts (most being M+ Keys) before it finally dropped.
I’ve heard that Heroic can take a LOT of attempts, but I’ve never had the patience to test that out, lol
This is correct. Also, your anima conductor will need to be upgraded to level 2.
I farmed 11 memories and purchased the other 13 using grateful offerings. It pays to click on the anima conductor daily to empower the locations.
Was far worse for me, I lost count a few days in, running that same dung on mplus as many times as I could find a listing…Started to think it was never gonna fall out of the sky 
The raid ones are 100% as are the world boss ones.
I think the dungeon ones are not, but I got mine pretty early (before the drop-rate was buffed). I believe the drop rate is better the higher the difficulty.
Gonna have to start now in any case, it’s an alt so I don’t even have that unlocked. But if it’s like that, I better do it now.