Vayne's Treachery would be a valuable addition to a black-based control or sacrifice deck that focuses on removal and leveraging sacrifice effects for value
With Lani [12-018H], for example, you can avoid so-called bad draws and instead take the most effective card possible from your opponent
Still, the opportunity cost of playing it is fairly low since it has good stats, but I just dont think this is something that you can prioritize
If youre buying booster packs or other physical Magic cards, I suggest hanging on to your hat until we get just a bit further down
Upgrades for BC: Take a good look at the creature base and figure out which ones are there because we wanted this effect on a legendary creature and which cards provide actual synergy
Knowing what those cards are becomes part of the information you have access to, just like you can look at the cards in your hand