I don't recall hearing about a drop of 21x1000 at SongSong but when I visited the range in 1964 the range guys told me about a possible similiar incident that could be the basis for the story.
At that time the V-Force had a requirement to drop two single 1000lb HE(S) bombs per year. HE(S) had a reduced spotting charge of 50lbs of RDX.
Anyway one night a Victor set off to drop his two bombs; one on China Rock off Singapore and one on Song Song off Butterworth.
He duly dropped the bomb on China Rock and it was reported 'heard but not seen'. Usually this meant the vis was poor and they range spotters were not looking where the bomb went bang.
The aircraft then flew to Song Song and dropped his second bomb. This too, with perfect viz, was heard but not seen. The crew had applied offsets from the Song Song island where the range party was to the target raft but had applied them in the wrong sense. Instead of dropping 3000 yards or so east they had dropped the same distance to the west.
The distance from Song Song to the mainland is less than 9000 yards.
Finally, someone might recall that the Victor could carry 35 x 1000 rather than the 21 carried by the Vulcan. This is true but the aircraft was underpowered and could not lift that quantity of bombs and a fuel load in the tropics.