Well, due to the growing pool involved, with just a party of 3 (2 vanguards, 1 singer) you could effectively expect 1 additional burst per round, which would tend toward the dice pool growing at a rate of +1, 3, 5, 7... per turn, for dice pools of 1, 4, 9, 16, 25. This works for smaller parties, but as the party grows larger, the burst additions would quickly outpace reasonable growth in encounter difficulty: With 5 in the party, 2 burst a turn is probable, leaving a growth of dice pools at +1, 4, 7, 10, 13, for expected pool sizes of 1, 5, 12, 22, 35. These are with 50% success rate on attacks. Over a duration of 5 turns, 35 dice worth of spell damage averages at 7 dice worth per turn. 7 dice per turn is overkill, I would think.
Possible solution (Leveraging in-universe fluff): The burst gauge grows via actions of a partner vanguard, the caster's chief defender, rather than from any vanguard. Sword and Song, if you will.