as you wrote above, from the CPU stand point, /IACK is what you wrote. On a ST, the GLUE takes this and generate an actual /IACK signals that goes to the MFP /IACK pin.
So the MFP triggers IRQ, which goes to the GLUE /MFPINT pin (level 6 interrupt). The GLUE then assert the IPLx accordingly. At this point the 68020 finish its current bus access and then goes into interrupt acknowledge (IACK) mode (FCx = 0b111 and A[19:16] = 0b1111). The GLUE assert IACK on the MFP, MFP puts vector on bus, CPU reads it, finishes IACK cycle and jump to vector. In this case AVEC is not asserted and all of this is part of the IACK cycle.
Now when an ACIA triggers an interrupt, the GLUE will also assert /VPA, in this case , the interrupt ack cycle will use AVEC as the ACIA can't generate vector.
From the user manual :
For a device that cannot supply an interrupt vector, the AVEC signal can be asserted, and the MC68020/EC020 uses an internally generated autovector, which is one of vector numbers 31–25, that corresponds to the interrupt level number.