Is there a NEED to deactivate VCM? "Need" is a very strong word, so the direct answer there is probably no. But the best answer is probably a little more nuanced than that.
It's really a case of different strokes for different folks - I hope the performance you're getting is now is what you will continue to get for as long as you own the van. Lots and lots of people do. But more than a few people don't, and this is for those who don't want fate or (mis)fortune to decide which group they'll fall into. Because it doesn't seem as though it's possible to know in advance whether or not you will have troubles. That's pretty much all it comes down to, at least until problems arise. The decision-making criteria change after being presented with a big repair bill. 😉
There is no doubt that driving habits have a major impact on fuel consumption, regardless of any other technology. Being smart, efficient and proactive on the road is just good driving, and I'm totally with you on that.