Why vectored interrupts are not possible?
In a configuration where multiple devices are tied to a single interrupt line to the CPU, and the interrupts are simply combined together without identifying the source of the interrupt (via some priority encoding, or perhaps a big logical OR), then vectoring isn't possible. It looks essentially as if there is only a single interrupt source.

Of course, interrupt dispatching can be vectored at some level. However, this "vectoring" is done by the software stack. There is a single main interrupt handler which has to interrogate all the devices to determine which one (or perhaps more than one) is signaling the interrupt, and dispatch the appropriate registered handler for each one.

Vectored interrupts means that you have some hardware support for distinguishing multiple interrupt sources, so that the actual primary interrupt dispatch done by the processor is already vectored. In the interrupt cycle, the interrupt circuitry actually hands the CPU a number which is then used to index into the vector table. The code which is dispatched can assume that the interrupt is for the device which is associated with that interrupt number. (Unless, of course, it is the case that in spite of multiple interrupt lines, interrupts are still shared! Say because therea re only 8 lines, but more than 8 devices that can interrupt.)