There is a difference between an Over-Mitt vs a Vapor Barrier Liner.
I will caution against using a vapor barrier liner (VBL) OVER the thermal layer of a glove, mittem, sock, etc. You will cause the thermal layer to dampen and lose insulative properties. Over mitt coverings, like goretex over mitts, allows sweat to escape. but waterproof materials will not.
VBLs either go next to the skin or on top of a very thin inner glove (or sock if it is for the feet) then the outer mitten (or socks) go on top.
For a Camino, I would not take expensive overmitts during most of the year. . . they are meant for prolonged activity in snow and ice. For their compactness, weight, and cost as a 'just-in-case' piece of kit, latex or nitrile gloves will serve quite well.
For feet, heavy mil plastic bags as a 'just-in-case' VBL are less likely to interfere with the fit of an existing shoe or boot than the more elegant solution of vapor barrier socks which can be a bit pricey. They resemble a foot shaped, waterproof stuff sack.
Another option for feet involve carrying a pair of 'waterproof' socks like Sealskinz'. If that is a 'just-in-case' strategy, than it is important to fit shoes or boots while wear this type of sock (and any other sock you might decide to wear on top of the Sealskinz).
I look at it this way:
Baggies, thicker plastic bags, nitrile or latex gloves, etc are temporary, 'just-in-case' solutions as adjuncts to the clothes-closet in my backpack. Over Mitts and VBL socks would become part of my clothes closet when known weather conditions make their frequent use a necessity.