This is a circumstance I'd view in the context of what is sacrificed vs. what is gained in cleaning it up. With iron + vinegar + water + air (oxygen, specifically) -> that's the chemical combination which produces new red rust from previously clean, healthy steel. The vinegar soak works to lift old rust while corroding the healthy steel underneath it. In circumstances where it's known some steel can be given up to clean it up, such as with rusted cast iron cookware, it might make things simpler.
But in the case of an old knife that's already assumed to be heavily corroded internally and it's not known how much more good steel remains, I'd view it as safer to do what can be done to just soften the old rust a little bit, which is loosely bound to the steel, without risking giving up good steel that might still be left behind. That's what the inert WD-40 will do - it isn't capable of corroding the steel at all, but it will help to soften the existing accumulated rust so it can be removed mechanically (by scrubbing). And the good steel exposed underneath it won't be put at any further risk.
I'd be most concerned about the springs in particular - the pitting induced by rust on the springs will weaken them, making them more prone to breakage.
The knife below is one that was traded to me years ago in the 1990s - it's an old Voss Cutlery 'springer' automatic, from Germany. It was found by a co-worker of mine on his property in Texas. It had been on the ground for an unknown period of time - probably a few years at least. It was rusted pretty heavily and the internal spring for automatic opening had long been broken and the broken remnant lost. So, functionally, it was just a locking slipjoint knife. I gave it a WD-40 soak as described above and otherwise just kept scrubbing and rinsing with more WD-40 until the red rust was gone. As it turns out, the knife was in pretty good shape and still functional. The springs inside the blade well showed the most corrosion and pitting - seen in one of the pics below. But it functions perfectly well as a simple slipjoint, locking folder. The blade took a screaming-sharp, new edge as well. And the stag is classically beautiful.
