Van Dough being so ignorant that he would assume the Rich family would keep hoarded wealth in their vault isn't that surprising. It's implied he's mentally a spoiled child who isn't good at business since, under the most basic scrutiny, his ideas are terrible for the bottom line in the long run.
Van Dough has zero concern for the welfare of the employees and gushes at the idea of downsizing and bottoming out wages. He treats his personal staff equally poorly: he fires a chauffeur over a minor mistake and takes his anger out on his secretary. In contrast, the Rich family treats their maids and servants well, while Richard's reasonable wages and benefits strategy ensures a highly productive and loyal workforce. Keenbean and Cadbury return that loyalty by stopping Van Dough from taking total control of the company.
Van Dough belittles the innovations Rich Industries produces as silly games. But once he almost assumes the company's reins, Van Dough's "innovations" are "trim-the-fat" yuppie capitalism that has nothing of benefit to society besides executive pay and some gifts for shareholders. Under Richard, Rich Industries invests heavily in neat innovations that, while ridiculous on the surface, could still have beneficial applications in the real world. The "Robo-bee" is a significant step forward in AI, and the smell machine has security applications. It's especially karmic that the latter device, which Van Dough mocks, foils Van Dough's plan to kill the Rich parents.
Van Dough also ignores the importance of goodwill. Richie and his father understand the importance of charity and goodwill since a company's reputation is an asset. Van Dough mocks the idea of charitable donations. Cutting them may save money in the short term, but in the long term, it weakens the company brand.
While it may seem on the surface that a McDonald's franchise in the house (with all orders no doubt charged to the Rich family's tab) is a largely unnecessary excess, the Riches' many servants and the employees of the science lab on grounds are likely grateful to have a cheap place to grab some food 24/7.
The movie's plot is about the Rich family being betrayed by one of their associates and head of security. While the film ends whimsically, the Riches will likely be in therapy over a traumatic event for a long time.
Richie is only spared being stuck in the difficult situation of Richard and Regina because his parents let him stay behind. If he hadn't gotten off the plane, he would've been stuck in the ocean with his parents with no chance of being rescued, and Van Dough would be in charge of the company.
Van Dough is willing to pursue ruthless business strategies and willingness to assassinate his employer and frame the butler. How many lives would he have destroyed in the name of profit had he managed to gain control of Rich Industries?
As stated above, Van Dough's business strategies are morally wrong and unsustainable in the long term. Within a few years at the helm, he might bring Rich Industries crashing down under the weight of his greed and incompetence, the same way many other corporate giants like Enron and Lehman Brothers were.
After succeeding in framing Cadbury for murder, Van Dough assumes custody of Richie, bans him from being involved in the company, fires all the servants of the Rich estate, and places a ton of surveillance and armed guards around Richie. One could assume this was to keep Richie under his thumb, but considering his willingness to murder the Rich parents, would he keep a tight leash on Richie so he could assassinate him? And would Van Dough have tried to murder Richie once he got old enough to assert control over the company?
Given how quickly Van Dough arranged for Cadbury's "suicide" in prison, it's pretty much certain Richie would get bumped off soon (maybe from an "accident" or another "suicide" over losing his family). Good thing he bailed as soon as he could.
