I've had good luck using a 3-way stereo crossover and a mixer. Run the stereo audio into the crossover. Connect the low outs into 2 mono channels panned to the center. Do the same with the hi outs. Connect the mid outs to 2 mono channels panned center with one channel signal inverted. (phase inverted)
Bring up the hi channels. Adjust the mid/hi frequency so the vocals are just hidden or extremely weak.
Bring up the lo channels and adjust the lo/mid frequency to just hide the vacals again.
Now bring up one mid channel and adjust the gain to match the his and lows. Then bring up the second mid channel to about the same gain and tweak its level to cancel the vocals.
This works because vocals are usually centered in the mix.
You will loose any mid frequency instruments that are centered. But lots of times guitars and other lead instruments are panned slightly left or right.
There are several vocal eliminator hardwares and softwares available also.
