That said, while they do use some complex formulas, they don’t use any VBScript or other such madness.
Complaining that your custom MSFT/VBScript stuff doesn’t work, so you can’t migrate — it sounds to me a bit like “We’re stuck on Oracle and can’t move to PostgreSQL because we decided to use a lot of Oracle magic stuff that doesn’t work anywhere else”.
This is classic vendor lock-in, which I tend to view as a business risk best avoided. There really is no option except to recognize it as technical debt and find time to dig yourself out from it.
When your only option for “Business Function X” limits you to a single vendor, fix that first.