From recent observatory studies, SIREN in UK , and in Denmark, they both estimate seropositivity to give around 80% of protection agains infection. The former observed 90% protection against symptomatic cases.

How efficacious are vaccines on the seropositive population? I've only seen reports on safety analysis. 
To the extend of suspecting malice, the Israel Pfizer study did not report on prior infection analysis - Study protocol states "Exclusion of patients with COVID-19 prior to the index date or matched index date."

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Related fact: Israeli vaccine policy has been to deny vaccines to recovered coronavirus patients for at least 3 months and then give only one dose. Despite the relative success of vaccine distribution there, there was still scarcity at the time this was developed, so this would indicate that the authorities judged the benefit (from a public health standpoint) of giving one dose to an immunonaive person to be equal to that of giving one dose to a recovered case after a 3+ month period.

I'm a little unclear on the biology, but its that just a matter of scarcity, or also a matter of expecting a stronger reaction/more side effects? I would think if having had covid gives you more protection than having gotten the first shot of a vaccine, then your reaction on getting a vaccine is likely to be stronger, and waiting might be valuable even just on that basis?

There are studies on vaccine safety on recovered, they seem safe.

But none has been published on the efficacy of seropositive population, which is not surprising when you estimate such a study population - would have to be 20 times bigger than the ones that are published.

It's both, if I understand your question. Having covid gives some resistance that declines over time (intentionally vague phrasing since research on this is ongoing) so recovered cases have less need for a vaccine and from a public health perspective offer less benefit than vaccinating an immunonaive person, if you can only choose one. In addition, giving people with strong immunity even more doses exposes them to needless risk, so recovered cases get only one.