NOTE: Spell/Skill MECHANICS will work as they did in classic. Example: Hunters will have the "dead-zone" where you can be too far to melee, but too close to shoot with their bow/gun. Priests will have their racial spells like vanilla (Dwarf Priests have Fear Ward and Undead Priests have Devouring Plague). All these things are calculated on the server, send to the client(s) of the players.
However, VISUALS will match Wrath (or HD) textures/animations, since those are from the client...so your graphics may not look vanilla, but the damage, cost, when your character has access, and all the other important details should match Vanilla.