Note that the Vietnamese only made 4 engagements against the SR71 in 1967/1968, and one was terminated without missile launch.
Of the 3 remaining engagements, against two separate flights, one missile fused and caused fragmentation damage... depending on whether you count the number of targets engaged, the number of firing systems, or the number of missiles fired, this is between 50%, 33% and 11% hit rate, but a zero kill rate. The only aircraft of this type lost were to operational accidents.

As we know, that the SR71 RCS is too small for burn-through over jamming (and HABU used to jam), I would assume, that Radio Proxy Fuse was armed by 11sec.