According to a recent study by Professor Cedar, Vaporean has the capability undergo variant evolutions tied to its environment.

This is speculated to be due to the highly volatile nature of Eevee DNA and the evolutionary tactic of extreme adaptability all Eevee possess. To adapt expertly to climate and environment upon evolution, the pre-evolution Eevee will adopt aspects of other residing species of pokemon, ensuring an incredibly high survival rate.

The standard Vaporean variant occurs if the Eevee has not resided in a particular environment for an extended time, or in heavily populated human city centres.