As far as I am aware of Flux does not care about what GPU you use, but rather what ROCm version your GPU can use. Considering that Vega 56 seems to support ROCm 5.x, if Flux supports ROCm 5.x, then Flux will work on Vega 56.
The question of GPU <=> ROCm version is a AMD question and the question of ROCm <=> Flux is a Julia AMDGPU.jl/Flux.jl question.
To find more about if Flux works with ROCm 5.x see the following thread: Flux with AMD GPU(s)?