You need to injure a player - that is the key wording. You don't need to tackle them. However, there is a special event that automatically injures tackled players - I have always played it that this also activates Blood Lust and you would still get the benefit.
Ah, now the wording makes more sense (we were confused about the 'tackle' part of the ability). I'd read it the other way though, you only get the token if you tackle and injure someone, so not with that event.
Obviously all of the above means if you get a tackle ball or a tackle coach then tournaments are your friend as you can easily got a SP5 vamp with the ball.
Yes, I had the tackle coach, unfortunately I also had the uncanny ability to only roll x and blank on most of my tackles when targeting opponents. I was quite good at eating my own thralls though... until my opponent figured out that he should injure them before I could.