There were U-2s based in Vietnam and in Thailand when I was there. My area was just south of Bien Hoa, east of Saigon when I was flying O-2s as a FAC. I looked out one day and a U-2 was slowly descending into Bien Hoa. I had angle and was high. It was just TOO easy. Pushing the throttles and props up I dove at the U-2 and was rapidly overtaking him as he was way back on speed for the approach. Then a light went on... maybe this was NOT the brightest idea I had ever had. Granted, I had joined on an RF-4 going into Tan Son Nhut (gear/full flaps) one day and all he did was flip me off. But this airplane was not even acknowledged as being in theatre. The U-2 drivers never showed up at the bar or if they did, no unit identifiers or patches on their uniforms. I pulled off the rejoin.
The next year in Thailand I got to know the U-2 drivers and the Lockheed rep. I was invited to hang around for a launch and got to sit in the old U-2. I commented it was quite roomy. The pilot said, "But you're not wearing the suit or helmet." They said I should apply and one or two said they would write a letter of rec. but if accepted, it added about 5yrs to the commitment. I said thanks but no thanks.
One of the guys at one FSI location flew them and we have had some conversations about the mission. He said that for the first year, as soon as he got airborne he began thinking, "How am I going to get this squirrelly SOB back on the runway??" He said he never felt completely comfortable landing it.