Re: Vetus Flexi coupling
You are correct that Nylock nuts a single use items (at least in aircraft applications and I assume in other critical applications). They are also low heat suitable as nylon deforms at temperature. I don't know how hot this area gets but perhaps metal locking nuts would be better. In the aircraft industry metal locking nuts are used in virtually all engine compartment applications. There are specially coated metal locking nuts that are suitable even on exhaust components.
Various types such as https://jlanfranco.com/products/all-metal-locknuts/ or for aircraft https://catalog.continental-aero.com/viewitems/all-metal-lock-nuts/all-categories-all-metal-lock-nuts-aero-flex
You will have to do some research to find the metric size and grade you need.
All the best,
Gary S. Silver
Amel SM 2000 # 335
Puerto Del Rey Marina, Puerto Rico