I lost all my vehicle saves as well, without any kind of soft lock though.
I was trying to launch multiple rockets and dock them in orbit:
built the first rocket, launched it from the VAB and returned to KSC
launched the same type of rocket again, this time from the launchpad menu, returned to KSC
built the second type of rocket, saved, tried to put it on the launchpad
Now here it started to go wrong: two of the four boosters were missing. When I reverted to VAB, the vehicle was in some intermediate state, not the state that was put on the launchpad and certainly nowhere near the state it was in when I hit the save button.
When I tried to reload the craft, all the saves were gone. Reloading the game didn't help. Surprisingly they reappeared, when I reverted to an older autosave
Why would the vehicle saves be related to different career saves within the same career!?