If a lee sin is getting one-shot by veigar 3 seconds after returning to base, that team has issues which far outweigh whatever assist sum the enemy vayne is getting. No system is going to be entirely perfect in how it detects how to reward gold, but I don't feel as though any such one-off scenario you can concoct is abusable in the current state of things.
Teams are rewarded more for working in cooperative force to secure kills rather than piggybacking off of random events of serendipity/utter stupidity which occur the current way. The reward for pushing an enemy back should be enough in itself. You gain more ground on gold, experience, and strategic pressure. If the enemy is dead and out of the picture for even longer all the better for you regardless of whether you have an extra tick on your score. That's when towers fall.