The short answer is that it is barely not good enough.
Compare Demonic Tutor to Vampiric Tutor. Basically everyone agrees that Demonic Tutor is better despite costing more and being Sorcery Speed.
Now add
and some life to the casting cost of each.
Demonic Tutor just became Grim Tutor which is sort of Barely good enough to see play. It basically only shows up in Long/TPS decks and then usually not as a 4of (Although Stephen Menendian recently suggested a Grim Long list with 4xGrim Tutor)
Anyway, the point is that Grim Tutor just became barely playable. Vampiric Tutor which started worse than Demonic Tutor just became Lim-Dul's Vault which is worse than Grim Tutor. This leave Lim-Dul's vault as barely not playable.
The comparison is not exact since Lim-Dul's Vault is not a straight top-deck tutor. It can help more than just a top deck tutor and it is blue and thus pitches to Force, but you generally pay more than 3 life like you do for Grim Tutor.
I would not be that surprised if a used for Lim-Dul's Vault were found and it were playable in some deck, but its general power is slightly less than Grim Tutor which is why I expect it to continue to be played less than Grim Tutor.
