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First, a disclaimer. I am no fan of vertical siding since it just does not do a great job preventing water from seeping into the wall as does horizontal, overlapping siding. I have seen too much wood rot encouraged by this visually attractive but functionally deficient siding. I don't have a place in my big heart for it!
In my opinion, caulking between the seams in any lapped siding (either horizontal or vertical) is not good practice but not just because of any potential vapor barrier effect. The siding expands and contracts with temperature and humidity. Think of a wood home as a living thing. It needs to move and breathe. When you try to restrict either it is at your peril!
Caulk, by its adhesive nature, will try to limit this movement, leading to widespread failure and cracking in the caulk over time. This will in turn be unsightly and not amenable to easy repair, adding to the prep workload when repainting. This is not to imply that caulk has no place in paint prep! Places where dissimilar materials meet and around window and door frames are two examples of areas needing proper caulking.
The other issue is the sheer volume of the caulk that would be needed to accomplish this job. And, barring the rental or purchase of bulk caulk and air compressor-powered caulking equipment, the time and energy needed to do such a job would also be prohibitive.
If the home has been properly constructed, and there is a "housewrap" of some kind under the siding to prevent exterior moisture from reaching the framing, you should not have a need to even consider any further weatherproofing outside other than a quality paint or stain job.
Aside... many people do not know this, but siding, shakes and clapboards should be pre-sealed on both sides prior to installation (or at least the inside), unless this is done at the factory. Once installed, it is impossible to seal the inside but it can nevertheless be victim to moisture and rot!