I did a bit of experimenting and think I've got a pretty good approximation the impact of "Growth" and "Sapping Vines." For the Growth trait, the table below shows the impact on Attack and Health. It looks like Attack increases by 5% (of the base Attack) for each stack of Growth up to a total of 40% at 8 Stacks, which is the maximum. Health scales up by 38% for each stack for the first 3 stacks, then 36% for the 4th stack. But it provides no additional benefit beyond 4 stacks. The maximum health benefit is 150% cumulatively. Note that these increases are applied on base stats plus any stats from runes, so runes do benefit from the buff.
-- Was the quick scaling of health intentional? It seems it would have been spread evenly over the 8 stacks, similar to Attack, but perhaps this was intentional.
For the Sapping Vines ability, it appears that it absorbs 20% of the all the enemies' (original) health and gives that back to your heroes as healing.
Also, it splits that total absorbed health evenly between all of your heroes. So for example, if you face 5 enemies and they each have 6,000 total beginning health, you will absorb, as healing, 6,000 in total each round for three rounds (assuming all the enemies are still alive at the end of each round). If you have 4 heroes on your team, the 6,000 will be split evenly between them and each will receive 1,500 in healing each round. If you only have two heroes, they would each receive 3,000 in healing each round. If some of the enemies die before the 3 turns expire, the healing you receive will go down correspondingly.