Silicone is commonly used as a water repellent and siloxanes are used as water repellents. Apply it your your wooden deck to repel water. Or concrete. Or clothing.
So the use of siloxane in cosmetics makes it last longer after application. Probably much longer than a vegetable oil. And the siloxane is probably much cheaper than the vegetable oil.
The ane suffix in siloxane means that is a hydrocarbon, like paint thinner or gasoline. It is a volatile organic compound.
Since siloxanes are a class of compounds and there are variations within the class, you really have to look at the particular siloxane compound you are using. Once you know that, you can look at the Material Safety Data Sheet, MSDS, to determine action taken upon exposure. I would expect the MSDS to say that, in the event of Siloxane exposure, the compound should be removed using soap and water.
You also have to recognize that since make-up is applied on regular intervals, then the exposure to the siloxane in the make-up would be considered to be chronic exposure.