how ludicrous an idea would it be to have a society running around in these helicopters without knowledge of the vacuum tube?
Very easy, very early discovery of semiconductors (empirical, because you need advanced quantum theory otherwise) and there is no need to develop vacuum tubes. This is actually rather easy in post apocalypse world, if (say) 19th century level society has access to some pre-war technical literature.
But that is not what you wanted to ask.
To miss the (re)development of radio, you need something else - working, reliable helicopter requires rather advanced and precise metallurgy, which presupposes big industrial and scientific base, and it's difficult to see how they would miss electromagnetic waves. Why a crude autogyro or even basic helicopter could have been build with 19th century technology, it would be ridiculously dangerous.
Instead, you might posit some external factor that makes radio communication impractical - perhaps a continuous electromagnetic storm that overwhelms any reasonably powerful radio communication. Perhaps caused by disappearing Earth magnetic field (geomagnetic reversal might be actually overdue by now) coupled with continuous solar flares. If your first experiments with radio waves show there is a lot of background static, the technology would not go along this path, since there is little practical use. Shielded coax cables will be still used for ground based communication, but light pulses will be the norm for distant messaging. Though, given reasonable advances in technology, the light pulses will be modulated for voice transmission.