For those VHS movies that are not currently on DVD or may not be anytime soon, you can try using a Macrovision Killer, which is a box that is placed between a VCR and DVD recorder (or VCR and VCR).
However, in the case of DVD Recorder/VCR combo, you need to check if the VCR section has its own set of outputs and if the DVD recorder section has its own set of inputs and that if the VCR can play at the same time the DVD recorder is recording, independent of the internal VHS-to-DVD dubbing function.
You would then connect the Macrovision Killer to the outputs of the VCR section and the inputs of the DVD recorder section. In other words, it would be like using the Combo as if it were a separate VCR and DVD Recorder. Your user manual should explain how to use your DVD Recorder/VCR combo in this fashion (minus the Macrovision Killer part) and offer an illustration.
Using a Macrovision Killer between a VCR or DVD Player and a DVD recorder, or between the VCR section and the DVD recorder section of a DVD recorder/VCR combo, may result in successful copy. However, this method may not work in all cases. Also, such devices are getting harder to find as a result of legal crackdowns.