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Vision Charm will change the following:

Lands' sub-type (allowable types listed in previous posts)

Vision Charm will not change the following:

Lands' super-type (Basic, Snow, Legendary)

Basic is a supertype. (as in Basic Land - Island). It is not a valid selection for Vision Charm. You can select one of the basic land types (Island, Swamp, Mountain, Forest, Plains) for the second...

Reading Vision Charm:
Vision Charm
Instant, U
Choose one — Target player puts the top four cards of his or her library into his or her graveyard; or choose a land type and a basic land type, and...

It would only affect permanents on the battlefield. Otherwise, it would have to refer to "land cards" and specify a zone.

No, this is not how Vision Charm works. You choose a land type (valid choices shown in the 2nd post) and a basic land type; each land of the former would become the latter. Desert is not a basic...

Correct. :u: and Island, and "blue" are not the same when it comes to cards such as Magical Hack. Blue mana will always be strictly referred to as :u: rather than verbally.

The only valid choices for land types are:

basics
Lair
Locus
Urza
Desert
Mine
tower
power-plant