The correlation between MSG and autism still need a long way to go, although there is a strong positive correlation between both. About being an excitotoxin, MSG contains a strong excitatory substance, which is glutamate, and therefore can cause serious electrical imbalance in the brain. But since MSG does not readily enter the brain, the effect on brain is minimal (except on elderly and young children, whose brain is easier to be infiltrated). However, that doesn't mean MSG is safe on healthy young people, since it damages the organ and causes several clinical threats, such as obesity, heart diseases, and cancer.
Vaccines, on the other hand, may or may not provoke autism. Some cases, especially live vaccines, can cause these or even exacerbate the disease it was intended to prevent, but only in a small number of cases (most of them are immunodeficient people). This risk is greatly outweighted by the advantages vaccines offers. Thus if it were given carefully, vaccines are proven to be safe.
Now about the the additive yet dangerous substances in food...the main booster is not only about the cost of the food, but more to the efficiency of time. People are getting busier and busier nowadays and to most of them, preparing homemade food are time-consuming and bothersome. They seek for instant food, which are ready all the time and doesn't need much time to be prepared. For example, to make a soup, you must prepare and boil stuff, prepare the stock, and then freeze it when there are leftovers. But with a canned soup, you can cook for only yourself by simply opening the can and cook it for a matter of minutes. The same thing applies to the usage of MSG, HFCS, artificial colourings, etc. Hence the ones which need to be changed is not only the substances, but also the people who utilise these substances (which has proven to be a difficult task since MOST people are too lazy to get out of the snare).