You're right, but I think the Texas court's analysis was flawed. The Texas Appellate court decision starts with the premise that the Texas Constitution on this issue follows the federal Constitution. "Having found no compelling rationale on which to justify interpreting the Texas Constitution as providing any greater protection than our federal constitution in the area of law enforcement searches to locate VINs, we overrule point of error one." Morgan v. State, 906 S.W.2d 620, 625 (Tex. App. 1995). But then the state court went ahead and drew a slightly different line than the U.S. Supreme Court did because it did not read the language of the Court's decision carefully.
In Class, the Supreme Court was reviewing whether it was permissible for the officer to enter the interior of the passenger compartment to find the VIN; and it held the officer was entitled to do that because dash VIN was partially blocked (and there was no door jamb VIN). The Supreme Court noted that:
The VIN, which was the clear initial objective of the officer, is by law present in one of two locations—either inside the doorjamb, or atop the dashboard and thus ordinarily in plain view of someone outside the automobile. Neither of those locations is subject to a reasonable expectation of privacy. The officer here checked both those locations, and only those two locations. The officer did not root about the interior of respondent's automobile before proceeding to examine the VIN. He did not reach into any compartments or open any containers. He did not even intrude into the interior at all until after he had checked the doorjamb for the VIN. When he did intrude, the officer simply reached directly for the unprotected space where the VIN was located to move the offending papers.
New York v. Class, 475 U.S. 106, 118–19, 106 S. Ct. 960, 968, 89 L. Ed. 2d 81 (1986)(Italics added.) Note that the Court clearly states, as indicated by the part I italicized, that simply looking at the VIN in the door jamb while standing outside the car is not an intrusion into the car. This is important later because the Court then rounds out its analysis with this limitation:
We note that our holding today does not authorize police officers to enter a vehicle to obtain a dashboard-mounted VIN when the VIN is visible from outside the automobile. If the VIN is in the plain view of someone outside the vehicle, there is no justification for governmental intrusion into the passenger compartment to see it.
New York v. Class, 475 U.S. 106, 119, 106 S. Ct. 960, 968–69, 89 L. Ed. 2d 81 (1986). Thus, opening the door while standing outside to scan the VIN, even if it was also on the dashboard, would appear not be a violation of the 4th amendment because that is not an intrusion into the car and there is no privacy expectation in the VIN. But if the VIN cannot readily be seen on the dash or in the door jamb the court held that a limited intrusion to view the VIN is permissible.
The Texas Court though somehow saw a line that the Supreme Court did not draw: that opening the door to look at the VIN is only permitted under the 4th amendment if the VIN is not visible on the dash. While there may be a good reasons for drawing that line, that is not the one that the Supreme Court made. But you are correct that in Texas as it stands now the rule is that opening the door to see the VIN is not permitted if it is visible from the dash (assuming no other exception would allow the officer to do that, of course).