Seems a perfectly sensible way to do a CDA. In fact, I'm not sure there is any other way. Just make sure the VS is appropriate wrt ATC requirements(many have a min 500'/minute requirement, although UK defines it along the lines of altitude lost per mile). Also making sure the VS doesn't override the speed required.
Maybe he was just having a bad day..............
In terms of fuel saving, I suspect a CDA does save a drop of fuel and they then multiply that by number of flights to make it more substantial. It's the kind of thing that works nicely in theory but is not really practical given the volumes of traffic that ATC have to deal with. Sort of like the optimum levels in a flight plan.......