Will a digital video stabilizer and video caption work on a vcr/dvd combo without a tuner?

It should work, yes, but obviously you'll only be able to digitise, stabilise and caption pre-recorded videos. Make sure your digitiser will take an input that your video offers an output for (i.e. RF or SCART). If it's a digital VCR (rare, but they're out there), you may want to export in a digital format for the sake of quality).