Normally when you cast Vampiric Burst, you gain an additional 50% of your max ammo, which can overflow your normal ammo limit. If Vampiric Burst is recast while it is already active (I tested this with massive cooldown reduction, not with a refresher), you do not get this additional ammo. In addition, the effect animation disappears based on the original end time of the buff, while the other effects (or at least bonus fire rate) seem to have a refreshed duration.