I’ve had the same problem. Before, it was a race, trying to stay ahead of my own roster. As a 4* hits 13/13, pulling a cover for them means it goes to waste (or you have to champ them to utilize that cover as a champ level). Thus, every 4* that hits 13/13 results in greater odds of pulling a 4* that goes to waste. The only way to counter this is to champion a character, which costs a lot of ISO. I was keeping track of my personal odds for pulling a 4* that would go to waste, and used that to decide whether or not to hoard CP and LTs or to spend them.
Then, Vaulting became a thing. Suddenly, it doesn’t matter if my Winter Soldier is 13/13, because I’m not going to pull a cover for him from an LT. So I didn’t have to worry. He didn’t count towards my odds of pulling a 4* that would go to waste, because he couldn’t be pulled in the first place. Vaulting wasn’t a system I wanted, or liked (I didn’t risk pulling a WS that would go to waste, but in turn I couldn’t continue to pull champ levels for people like Star Lord or Cyclops that I actually liked to use), but I was at the right spot in the game that I could make use of it. Newer characters got covered faster, and the odds of pulling wasted 4*s went down.
Except now it’s caught up. I’ve covered the newer 4s well enough that my risk of pulling wasted 4s is jumping back up. I really want to keep pulling LTs because I want more Iron Fist (and the same will happen when Cloak & Dagger enters LTs), but I run the risk of pulling one of the two Wasp colors I have at 5 already, or another Riri/Mordo (since they’re at 13/13, but unchamped). The ISO just isn’t enough to keep up, even pulling in 50k a day, on average.
Further, this system punishes me if I pick poor choices to champ. I’d like to champ Iceman, or Punisher, or Moon Knight (all at 13/13), but the reality is that champing any of them costs me a lot of ISO, but doesn’t increase the odds of pulling usable covers from the LTs. So if I champ the likes of them (or any of the 9 other vaulted 4s I have at 13/13), it’s going to lead me to start hoarding CP and LTs again. My other option, if I want to keep pulling LTs, and keep the odds of getting usable covers as best as I can, is to invest ISO into Riri or Mordo (the two 4s in packs I have at 13/13 but not champed).
Vaulting says “If you champion one of these high quality old characters, you will be forced to pull useless 4* covers or hoard until your ISO catches up; OR, you can champion one of these far less exciting characters, so that when your 1/12 odds inevitably gives you more covers for them, at least you can scrounge out the meager champ rewards.”
If we could rewind, even a 10% boost to ISO (across the board, in the entire game) would be a way better solution, because it would let me advance as I choose, instead of being bound to planning around the contents of Legendary Tokens.