If you have sufficient flexibility at the area in the photograph, it would be a good idea to add a mechanical reinforcement to the joint. Add an internal sleeve to the smaller portion, which will restrict movement to allow it to remain "within reach" of the outer portion. The remaining gap could be filled with silicone caulk with a lower possibility of the segments pulling apart. You would want to ensure a clean surface on which to apply the caulk. If you have excessive movement due to thermal expansion and contraction, the caulk may also separate.
If you can find PVC pipe with an outside diameter of the inside dimension of the smaller portion, stuffing a few pieces of the pipe should work. The verticals of the railing extend into the smaller portion and would serve as a stop for the sleeving.
There is vinyl glue available for securing this type of material. I've used it on my vinyl fencing with limited results. For your region, check the temperature restrictions on the packaging to ensure it will function in the cold. It may hold well enough but might be be able to be applied below certain temperatures.