Hi Norbert, I am not sure, but it might be related to this discussion on the fontinst mailing list: http://www.tug.org/pipermail/fontinst/2016/001692.html
No, Igor is reporting something unrelated, purely in LuaTeX's handling of vf files. The fontinst issue that Michael and Lars were dealing with was specifically about fontinst and arithmetic conversion in TeX of the designsize value from the vpl (bad) vs. keeping it as a string (good). I hope Lars releases a new fontinst with the fix. -k