Don't want to write this by hand? GodCraft sets minecraft:behavior.vex_random_move for you with a switch.Get GodCraftOverview
minecraft:behavior.vex_random_move is an entity component: it goes in the components object of a custom mob, or in one of its component groups. Makes the mob fly to random spots nearby, passing through blocks, the way a vex drifts around.
Example
Inside the components object of BP/entities/<name>.json (values are the defaults or placeholders):
BP/entities/<name>.json
{
"minecraft:behavior.vex_random_move": {}
}