vivo's fast charge is slow on Xiaomi because the two use different fast charge technologies and standards, and there are compatibility issues.
Specifically, vivo uses its own unique fast charge technology, while Xiaomi uses its own fast charge standard.
The technology and standards between the two are not the same, so there may be a mismatch between each other when charging, resulting in slow charging or no charging.
In addition, different brands of mobile phones may also have differences in hardware design, power management and other aspects, which may also cause fast charge compatibility problems.
Therefore, if you use vivo's fast charger to charge Xiaomi mobile phones, it may lead to slower charging speed.
To sum up, vivo's fast charge is slow on Xiaomi due to different technical standards and compatibility problems.
Charging current amperage and voltage are different