S-125 Neva/Pechora(SA-3) is not designed to destroy vapour, but airplanes. This system can be guided with several types of radars>
P-15 "Flat Face" or P-15M(2) "Squat Eye" 380 kW C-band target acquisition radar (also used by the SA-6 and SA-8, range 250 km/155 miles),
SNR-125 "Low Blow" 250 kW I/D-band tracking, fire control and guidance radar (range 40 km/25 miles, second mode 80 km/50 miles),
PRV-11 "Side Net" E-band height finder (also used by SA-2, SA-4 and SA-5, range 28 km/17 miles, max height 32 km/105,000 ft),
"Flat Face"/"Squat Eye" is mounted on a van ("Squat Eye" on a taller mast for better performance against low-altitude targets), "Low Blow" on a trailer and "Side Net" on a box-bodied trailer.
Radar screen showed object size - similar to wild duck, but one thing was strange, duck usually don't fly at 400kts. Radar picked it up, lock and 2 SA-3 missiles was launched and one "back up" SA-2 from another unit, soon target was destroyed. When you know what to search, you can manualy put search sight on target - rest is up to missile.
And, with all respect Fubar, you were not there, but some of us were.