First off, define brackish. IME, few to no FW plants do well at all if they even survive past 1.003. A tablespoon or so of salt per 5 gallons just ain't brackish in my house.
Vals do seem to be able to adapt pretty well, but the acclimitizarion needs to be gradual. I start the tank FW with buffering substrate, and increase the salt very gradually over multiple weeks.