I think I know what's the problem but I can't figure out the why. Either way I can offer a solution or rather workaround to the vector issue.
The problem is that I got the 1 in a billion of a savegame seed that causes these crashes. And it's rather picky as well by choosing which terrain blocks/alien deployment it wants to crash with.
I have found the seed value in the savegame and I tried to toggle a few bits. Toggling the LSB didn't change the behavior while replacing the seed value with another number did. I am not sure if Warboy would appreciate me spilling the beans, so I am gonna leave it there.
As for the solution/workaround, assuming someone gets such a seed and plays without savescumming, this is the way to fix it:
Enable savescumming, head to a tactical mission (important!), make a new save, disable savescumming, continue playing.
What are the odds of getting a problem such as this...