I don't understand the notion of opening a subform in a popup window/form. The subform for vehicle details should be based on the VIN field from the main form and that subform should be on the main form. The VIN field should also be part of the subform so that the 2 forms are linked via Child and Master linked field properties. Whatever VIN is showing on the main form, the related maintenance records should be showing on the subform.
The more we hear silence, the more we begin to think about our value in this universe.
Paraphrase of Professor Brian Cox.