The official documents are not clear about the interrupt vector provided in mode 2, I found the following are correct:
{| class="wikitable"|-! Interrupt Source !! Vector|-| DMA Channel 0 || 4|-| DMA Channel 1 || 2|-| DMA Channel 2 || 0|-| Raster Interrupts || 6|}
The interrupt vector is therefore ((IVR & 0x0f8) | Vector).