So if a vastaya's powers and appearance are highly dependant on the surrounding magic...
... does that mean all of Nami's skins (save for Urf-the namitee which is obviously a costume) are canon and she changes her appearance if she spends a long time in a "new" enviroment? Like, being to long in the abyssal depths turned her into the "Deep Sea" appearance, spending time in the rivers of Ionia has given her the Koi-appearance, and being close to Kumungu for a long time could make her look like in her pretty River Spirit skin?
And obviously if she is in a "normal" area or one with a low concentration of magic she would slowly revert into her "usual" appearance.
Note that I am not talking about a sudden change here, that could only possibly occur if she was hit by a LOT of magic at once, and that sounds quite unhealthy for me, even for a species that lives from it. Like, I need to drink water to survive, but drowning is still a problem for me for obvious reasons.
What I am mostly talking about is a slow adaption over the course of weeks or even months of being exposes to the "new" kind of magic, and tbh that would be kinda awesome.