Walking Ballista has at least two +1/+1 counters on it.
Steps
Activate Walking Ballista's second ability by removing a +1/+1 counter from it, targeting itself, and putting two +1/+1 counters on it due to Vigor and Uncivil Unrest.
Repeat step 1 as desired.
Activate Walking Ballista's second ability by removing a +1/+1 counter from it, dealing 1 damage to any target.
Repeat step 3 as desired.
Notes
Once Walking Ballista has three or more +1/+1 counters, you may instead target another creature you control using step 1 to put +1/+1 counters on that creature.