For the most part, GW removed all of those 'immunities' from weapons and units in the codexes for just this reason.
There is nothing, as far as I know, in the GW rules that defines 'unliving' as related to the Vexanthrope. It is a piece of 'fluff text' in the rule. The only thing to look at with regard to the target unit is whether it has to take a LD test (or is immunue based on its special rules), or whether the weapon specifies exceptions.
Anything else is using fluff to justify rules and vice-versa, a bit no-no
In the case of the Vexanthrope, it is very clear:
"Vexantrope cannot affect daemons, vehicles, Talos, Tyranids or walkers."
Nothing more, nothing less. Necrons are not listed.