A little update for those in similar needs:

I used Cascoflex without removing any old glue (neither from the old vinyl nor from the hull), just applied fresh Cascoflex with a paintbrush.
Works just wonderful (even when glueing the vinyl to overhead surfaces) and sticks perfectly.

Regards
Uwe

Thank you so much for indicating the exact product you used,
I could even locate a dealer in Germany who carries Cascoflex 3442!

Uwe

Thanks for all the valuable input, this sounds encouraging vs. removing all the glue! @M26 397 Helena: As I have already tried the "Pattex" brand contact glue with no success at all I was wondering which brand/type was the one that worked when you used it on top of the old stuff ? "Vatrumslim ...

Dear Forum, the vinyl headliner on my IF (1974) comes off the hull in various locations. The greenish glue on the hull as well as on the vinyl is still sticky but does not hold anymore. As far as I can tell it is still in the original "configuration" - has not been redone before. I was wondering ...