If you're playing a home game, then chained Rogue Talents are subject to GM discretion and you can simply ask if you can take the ki pool talent. If your GM says no, I doubt he'll be receptive to the technical workaround you're proposing. Another option for a home game is the Eldritch Scoundrel archetype, which gains spellcasting and the ability to spontaneously convert spell slots into ki.
I don't believe any of these options work in PFS. The official ruling is that Eldritch Scoundrel is incompatible with the Unchained Rogue (no reason given; I don't see any basis for this in the rules) and the Unchained Rogue is not allowed to pick talents from the Chained Rogue list, period.
So ninja trick's selection of rogue tricks is limited to those available to the rogue. So you can't get chained talents on unchained that way.
If Vanish is what you want then take the two magic talents and take the Vanish spell (as well as a zero level spell).
Vanishing Trick is a swift action to activate, so it's significantly better than Vanish as a SLA.
It may be better, but if he REALLY wants it he has little choice.
Take 2 levels of a Teisatsu Vigilante. [or just switch over to it]