@Morghan,
It seems you are using PowerApps custom form, and the Validate message is not available for PowerApps custom form.
You can set the validation and message in PowerApps. Please check:
1. Set the OnSave property of SharePointIntegration, add an if condition before patch function.
If(Over500Toggle.Value && IsBlank(NameOfVPDataCardValue.Selected.Value),Set(Vartext,"Required"),Patch(...);ResetForm(SharePointForm1); RequestHide())
2. Set ErrorMessage of NameOfVPDataCard:
If(!IsBlank(Vartext),Vartext,Parent.Error)
Then if the toggle is selected, but the VP is not selected, the save button won't work, and show the error message.
Sik