Vintage and retro are two terms with many overlapping meanings. For some people, these words are defined specifically, but other people use the two terms interchangeably. Generally speaking, vintage refers to objects that actually are from a certain time period, whereas retro refers to objects that are associated with the design ethos of a certain period whether or not they were actually constructed during that period. Some people define vintage as having a specific date range, but in most cases which objects are considered vintage depends on the date. Both terms can be used both seriously and sarcastically for different items and can be applied to many different types of objects.
The difference between vintage and retro as descriptive terms is largely a matter of opinion. Some people claim that retro items are never old items, but in practice, this is not always the case. It is true that for an item to be vintage, it typically must have been made in a specific time period before the present day. Even so, these vintage items are sometimes described as being retro when worn or used. As such, the terms do overlap in practical usage.
Retro is often used to describe items that appear to be from a particular vintage period but are actually new. For example, a reproduction of a vintage chair would be retro. Vintage and retro are therefore very different in this sense, because a new item is never described as vintage.
Essentially, the best way to think about the difference between retro and vintage is that vintage refers to construction, while retro refers to appearance. This is why a vintage item will always be retro, but a retro item is not always vintage. For practical reasons, people sometimes prefer to use retro only to describe new items, as this prevents confusion between the two terms. Even so, this is not a solid rule, and not everyone abides by this separation.
It is important to note that vintage and retro can also be used synonymously in sarcastic instances. Vintage items are typically items that were made within a particular time period, but that specific time period must be newer than antiques and older than items that are merely old and out of style. When an item is merely out of fashion because it is perhaps 10 years out of date, it is often said to be vintage or retro sarcastically to point out that it has not yet become fashionable once again.