Depends.
For Mythic raiding, yeah, probably ; we didn't went far enough during Vanilla to be actually able to make relevant comparisons (comparing MC to Mythic is pointless, it's Naxx40 and maybe the second part of AQ40 which would be relevant), but I'm ready to accept that mythic raiding is harder than Vanilla raids.
For leveling ? Vanilla is harder by a mile. Not "hard", but "harder". I need to pay attention in Vanilla, I can actually die, and I often have to actually use my skills (both as a player and with my character) to survive some encounters. I don't remember anything like that in current WoW.
For dungeons ? Comparisons are difficult to do, because Legion is more about rushing (timer) while Vanilla is more about simply completing the dungeon. Also, what would be the adequate comparison ? Normal ? Heroic ? Mythic ? Mythic +2 ? Mythic +5 ? Mythic +10 ? Add to that the extreme power difference due to gear (in Vanilla, your power in full T2 compared to your power in full blue is maybe +20 to +30 % ; in Legion, your power in full blues compared to Nighthold is what, +400 to +500 % ? More ?), it makes any comparison even less significant.
PvP, I don't really know and I simply don't care. I hate PvP
It's somewhat true, but I think it misses very important stuff.
Like "the easy stuff" in Legion is EVERYTHING but Heroic/Mythic raiding and high-end Mythic dungeons. That's more like "everything in Vanilla is harder, save for the fringe top 5-10 % of content".
Also, the TYPE of difficulty is different. Legion is all about "do your rotation and move". Vanilla is more about "manage the fight". Legion is very "self-centered" (you're focused on what YOU do and don't really give a shit about what others do, and nearly every change to the game since WotLK has been about removing other's effect on your performance and concentrating everything in each player's hands), while Vanilla is very "group-centered" (you depend on others for a lot more, and managing a big fight involves depending on what others do, like typically threat).
Legion general gameplay is also nearly entirely focused on "output" ("how much healing do you do", "how much damage do you do", "how much damage can you take") while Vanilla is more about situational awareness/skills and, again, "management". There is little gameplay maximizing your damage ("rotations" are very simplistic) but a lot of gameplay to mitigate shit hitting the fan (crowd control, slowing down foes, managing threat and mana, etc.).