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.
If the main upgrade (M/MK) is in 1969, I think that Vietnamese, couldn't do anything to SR-71, and this damage is highly uncertain... With just 2 G's over 20 km, in "T/T" mode.... I am very sceptical it happens.