While Vinyl Siding itself is waterproof leaks can happen. They are usually caused by openings at the joints of the siding sections, drain openings, bottom edge and not having a waterproof barrier.
House wrap, door, and window flashing are also areas to inspect when leaking occurs around the siding. If they are not properly installed the water will penetrate the siding and enter the building structure. This can lead to rot, insect damage and mold.
Wind-driven water is a major culprit of leaks and unfortunately, caulking will not stop the water from penetrating through the siding. The water can be forced into gaps around unsealed window and door trims, as well as into the overlapping ends of the siding pieces.
In the winter months, if you start to see stains appearing on the siding, its a sign of the moisture migrating through your walls. It is condensing as frost, then melting again once the day starts to warm up. This would not cause a need for new siding to be installed but more likely a ventilation issue that needs to be addressed.