Greetings.
I assume (from context, correct me if I'm wrong) that you're using MZ, as you mention the VS plugins specifically. Unfortunately, my dual wield plugin doesn't split damage in MZ anymore, as a result of changes made to a certain version of the VS Battle Core plugin, namely that VS team decided to include their own version of dual wield damage splitting, and the obfuscation prevents my plugin from knowing the exact formula they use for the offhand damage calculation.
With that being said, the Weapon Attack formula plugin also cannot affect the offhand weapon swing differently than the mainhand swing, because I have no way to know how VS is calculating the damage of that offhand attack anymore, nor do I have any way to modify the formula for one hand specifically.
So, in conclusion, you're not wrong, it is ignoring the weapon formula of the offhand attack. But it's the VS BattleCore plugin that is ignoring it, and the MZ version of the dual wield plugin can only do damage splitting properly if you downgrade your version of the VS_BattleCore plugin to a much older version, and downlaod the older version of dual wield from the devlog.