I'm on KSP 1.3.1, I've unlocked the Vista Fusion Engine in the tech tree quite a long time ago, and I've been using it on several different crafts. When I updated to KSPIE v1.20.13.0, a bunch of the crafts that I have saved (that I've already used a lot) say that they can't be launched because the Vista Fusion Engine is not unlocked.

I have to admit, I was pretty drunk when I tried it, but as I recall I actually tried that and it still wouldn't work. I have a lot of spare science laying around not being used, so I just went to the tech tree, and found it again, and unlocked it... but the VAB still showed my crafts as having an invalid/locked part. Also, the Vista Fusion Engines which I already had it orbit wouldn't produce a plume (they just kinda glowed blue) and didn't make any noise, but they did produce thrust.

I've reverted back to an older version of KSPIE, but when I get a chance to do more (sober) tests on the 1.20.13.0 I'll get back to you, most likely some time tomorrow.

Because of high performance, the Vista Engine tech requirement have been significantly increased. You simply have to unlock the tech to make use of it again I'm afraid

Confirmed. The Vista Fusion drive is owned in the tech tree, but I am still unable to use it.
Apologies for the delayed response.

It also appears that the lack of plume was not an issue with the new KSPIE; when I reverted to version 1.19.8, the problem did not go away. I'm beginning to wonder if that's an issue with RealPlume, so I'm going to attempt to reinstall that. But for the time being I've no choice but to revert to KSPIE 1.19.8, since that was the last version that didn't have this "unlocked yet can't use the tech" bug.

Ok, I've replaced every Vista Fusion Engine on every craft that uses it with the new version, and they're all unlocked. However, I launched one of these crafts, and I'm still not getting a plume, nor any audio, from the Vista while it's operating and producing thrust. It's the only engine I have which is having this issue. I've tried uninstalling RealPlume and the issue did not go away. I reinstalled RealPlume and the problem still hasn't gone away. I'm going to close this issue and open a new one. Thanks for the help.

the plume issue with RealPlume can be fixed by slightly altering the realplume module manager script file by changing vista into vista3

Can you point me towards the location of this file? Can't find any references to KSPIE in the RealPlume folder, and I have two Module Manager configs in GameData (ConfigCache and ConfigSHA)

Ah, I think I've found the problem.

I still have the Vista engine in my parts list in the VAB. These old ships are saved with the "old" Vista engine. If I open each individual craft and replace their existing engines with a new one from the parts list, then they become unlocked. This is going to require some tedious work, but I think it solves the problem.

I wonder if this will also solve the plume issue. I will have to perform some tests.

WarpPlugin\RealPlume\Vista.cfg

@part[vista3]:NEEDS[RealPlume,SmokeScreen] //D-T Vista ...?

That didn't seem to work

Or did you mean change Vista.cfg to Vista3.cfg