Sin's last ability doesn't cause you to move counters from Sin onto the target creature
Even so, the odds of pulling Thought Vessel in a deck of 99 (excluding the commander, who goes into the command zone) are slim, so adding Reliquary Tower can increase your odds of having a limitless hand size
It's not the worst stat line in the world, cards like this saw play back in the early days, but the game has moved on to better things now and so should you
Stuff like Snuff Out or Dismember are already heavily played cards, but here, you get to draw a card from them because you lost 4 life, and you also get to drain your opponents for 2 life, offsetting your life loss
asking a quick question is always faster than untangling a misunderstanding later