I want to emulate the Vai pickup combination. What kind of single coil should I use?

Evolution® Middle single-coils that come on the Ibanez JEM are not ordinarily stocked at retail outlets, though some online dealers carry this model as the ISCV2. You can achieve a similar result by wiring an HS-2™ for single-coil operation only: black wire hot, red and bare wires ground, green and white not connected.