Very nice pattern! Yes, in each of the first 8 rows you worked 3sts fewer. Now you're going to continue working 3sts fewer in each row before the turn. So the next row is 18sts, turn and work back, 15sts, turn and work back, 12sts, turn, and so on until you get down to 3sts left. Then you want to work across all the sts for 2 rows before repeating the sequece from row 1 again.
Sounds like very pretty shaping on this vest.