There is no specific spot on the field, as far as I know it merely has to land somewhere on the field, I think it can be close, but I don't know what the range is.
As far as I know you cannot resupply a field from its own VH, the shipment must come from a different field.
I don't think there is a text buffer message confirming resupply, I've always done it visualy.
And lastly, No, you cannot specify what targets get supplied, they all do anyway.