I haven't seen toons attacking each other in PvP yet, fortunately.

And reading the comments it seems I'm one with great luck regarding sacrifice - got rid of 1* souls and filled vortex twice with a VI and a VII sacrifice - reward count so far (including achievements):

1 skill shard
17 legendary souls
2 Magus souls
1 Warrior Soul
10 G/A
19 Rare souls

Still have some 4-5* dupes though, will see what the end result will be

For the time being I see that it is worth leveling to max your 3-5 star characters. The rest are just for feeding the vortex meter.
I did have some sacrifice freeze issues, but did get the rewards nevertheless, as Sparton told for others