VS has several flights within the Caribbean that are "via" flights that stop at another Caribbean airport to pick up passengers before heading back to the UK. SJU-ANU is one route, for example, that is part of a larger SJU-ANU-LGW route. I suspect these via routes only carry connecting traffic, where the passenger will ultimately head back to the UK but is using VS as part of their trip package to hop from one Caribbean destination to the next.
Note: Of interest is a route to/from SJU, which is not listed as a VS destination but VS somehow has scheduled service there.