Are synthetic vitamins any different from natural ones talking about their actions in the human body?

For example, vitamin C is l-ascorbic acid, irrespective of whether it is prepared in a laboratory or found naturally in an orange, it is still (and will always be) l-ascorbic acid.

The difference between synthetically produced and naturally occurring vitamins is not down to their chemistry, but more down to the way that the substance gets into the human body. Naturally occurring ones are already bound up as part of the food, so eating the food gets the vitamin directly into the bloodstream along with other nutrients in the food.

Synthetically produced vitamins are (usually) delivered in the form of a capsule or tablet, and whilst they can be taken with food, the tablet has to first dissolve and then the contents needs to be broken down and absorbed through the stomach. This can take some time, and there is no guarantee that all of the vitamin will be fully ingested.

So its more down to the delivery system than to the vitamin itself. But chemically speaking there is no difference in the molecules between "naturally occurring" and "synthetically produced".