About salt and vinegar:
Vinegar is biodegradable and won't last long in soil, let alone build up.
Salt accumulation is a well-known hazard for irrigated farms even where the water contains only traces. Prevention and cure is flushing the salt out of topsoil. So salt accumulation is most likely where the climate is dry and soil drainage poor. If drainage is good, growers sometimes flush built-up salt down by flooding.
Personally, I'd never apply salt to a garden.
I've had a vineyard and used mulch fairly successfully to control weeds, but that was in a land free from kikuyu grass. What ***** brought that in?
Whether plastic mulch fits your definition of organic, it would be effective, except kikuyu would probably find an opening next to the emerging hop vine.