Just as a general observation, I doubt it is official US policy, nor, it can be argued, is it in the US's purview, to give US Citizens vaccinations worldwide.
Another reason probably is, US Embassies are not designed to function as hospitals/clinics. They also probably do not want to try to duplicate the health services of their host country which probably already has some program in place for foreigners.
Some other reasons: Embassy personnel are easily counted, they know how much vaccine to send for each embassy. The State Department and other US Government entities can dictate to their personnel, when and where they MUST get the vaccine. Those criteria do not apply to US Citizens living abroad - there is probably no country in which the US knows exactly how many US Citizens are living there and exactly where they are, nor can they order them to queue up for any vaccine at a given time and place - and many of the vaccines must be used within a certain time frame or dumped as unusable. I think everywhere they try to use the vaccines as effectively as they can and not waste them.
In any case I don't think it's accurate to say that US Citizens can't receive vaccines from the US Government - you just can't get them overseas, as far as I know. Come to the US and you'll get a vaccine pretty easily, I believe.
Update: Here is what the US Government via the embassy in Egypt says about the issue, so far. Presumably this is official policy worldwide - I have seen the same or similar information on US Embassy pages for other countries.
https://eg.usembassy.gov/covid-19-vacci … ar-issues/
And specifically for Paraguay, here's all I could find from the embassy there:
https://py.usembassy.gov/covid-19-information-2/
