The roof however will need to be properly cleaned free of the moss first. There are companies that do just that type of work. Just remember that if your roof is getting up there in age, you may end up needing a new roof as well.

The fix is to remove the area much like you would remove vinyl siding. There is a tool with a little hook on one end that reaches between the joints of each section and disconnects one from the next. Once removed you will see, or should see the Start channel that appears to have come loose. Re-nail it or screw it back into place. Just be careful not to hammer or screw to tight. You will see slots where the nails/screws would go. This is to allow expansion of the vinyl when it gets hot.

You may need to remove several sections until you get to a point where the vinyl is secure and not sagging. Then simply work back to the end again.

Its not a hard job, but sometimes its a bit tricky if you have not worked with vinyl before. This may be a job for a handy person or siding company to do.

Just be sure that you do NOT nail through the surface as this will not allow for the siding to expand and the result will be all sorts of bowing when it gets hot.

Your only concern here is to see how much of an overlap was done when it was applied. if you can see the tar paper, plywood, or what ever was put on the wall before the siding went up near the top edge of the butt joint, the only proper method to fix this is to slide flashing up behind the joints.

If you still see the under course of the siding and its going up beyond the bottom of the course above meaning you cannot see the top edge of the board below I would simply clean out the old caulk and not worry about it as the bottom board is protecting the wall.

The bottom line is there is no sealant unless you coat over the top of each board, which would be ugly that will flex enough and still hold onto each edge of the board without adhesion failure.