Previously when consdering Forbidden Lore, I shied away because I worried about losing the ability to cast IoN, but after rereading the IoN text, it is outside the Vampire Lore, and therefore is known instinctively even if you take another Lore.
Maybe this was obvious to some, but it just occurred to me. Zoiks!
It says specefically in the Forbidden Lore text that the lore chosen is in addition to ION.
Which only makes Forbidden Lore that much more of great ability. Not only can we choose any of one of the Lores (except Life) from the Rule Book, but we also know all them and get to keep Invocation. You get 7 spells to play with every magic phase.
My favorite Lore right now is the Lore of Fire. I blast the enemy with all the destructive spells then try and launch my Lord into them with the Flaming Sword of Rhuin (+1 attack, all attacks hit on a 2+ and +3 strength). Pretty sweet if your Lord has Red Fury (Y) .