I find your answer reassuring. Actually yesterday I had another go at the Instrument in question, and there were no problems whatsoever. However, I am going to be doing a lot of this work in the near future, and if any pattern emerges (or even if it doesn't), I shall be in a position to make some kind of fuller report.
The successful work I did yesterday (using Windows 7) was done using Viena 0.780, because I had the feeling that it was more robust in what I am trying to do than the recent versions. I also had the feeling that the problem was more to do with importing samples with the same name as existing ones. On the other hand, the problems seemed to surface initially when I was trying to reassign a particular sample (using a recent Viena version). It was at the top of the reassign samples list, but FOR THAT ONE SAMPLE, when clicked on, the "OK" was greyed out.
Let me take this opportunity to thank you for Viena. I think it is an amazing program, and it, along with other computer developments, is allowing me and others to achieve results we would never have imagined possible.