Beetle aerodynamics (full-scale streamlining mockup of original Beetle)

[aerohead]

The windshield and all fenders will contaminate the flow for the whole car.
Breer tried to compensate for this.

[freebeard]

The early windshield had a generous radius surrounding the stagnation on the minimal flat screen. Later they prioritized the driver's sight lines.

Agreed the fenders add frontal area and contaminate the flow.

[aerohead]

The pre-1975 Beetles had a 31-degree laid-back windshield. While flow 'IS' attached over the header, it's got a terrific suction peak.
The Cd 0.196 ROVOMOBIL has a 48.5-degree windshield.
Hucho published research which clearly demonstrated an aero advantage downstream of windshields, when the glass was raked back to greater angles, limited to around 65-degrees because of refraction and internal reflection issues which would compromise forwards outward vision.
If you could rake the glass, it would pay dividends.

[freebeard]

Heavy rain can produce a rooster tail off that windshield.

Would a row of Air Tab vortex generators provide any benefit?

[aerohead]

I don't think so.
The windshield header is already submerged within a favorable pressure gradient, as far back as the roof apex.
And, as far as the CFD suggesting that flow is separated there, I don't believe that to be the case.
It's just a very strong suction peak, and very strong pressure spike, which lowers the ability of the boundary-layer, rear of the roof apex, to withstand the adverse pressure gradient of the fast falling roofline, triggering the flow separation we see in Volkswagen's wind tunnel photos.
Hucho wrote a comprehensive explanation of how beneficial it is to 'soften' that area of the body, with respect to its affect on flow adhesion downstream.
Dr.-Ing. Eberhardt, who designed ROVOMOBIL was no doubt aware of what his 48.5-degree windshield slant could do.
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The rain, 'over-shooting' the header is more about the inertia/momentum of denser water, compared to air. The air would have no trouble rounding that intersection.