I don't think any ASIC vendor is mining ETH currently, because of several reasons.

Firstly, Ethash is designed to be memory-intensive, and that means that the ASIC would be very costly to produce because of the amount of memory that needs to be addressed by every single chip.

On top of that, chip manufacturing technology is stable at 14nm, with some players trying to jump to a lower and much more expensive 7nm production process. Popular GPUs use 14nm technology, so any ASIC that pretends to be more efficient must be lower than that (Bitmain Antminer L3+ is 16nm, for instance), thus becoming very expensive.

I don't discard completely the possibility of Ethash ASIC mining, but I think that nowadays the more efficient option are GPUs. The difficulty ramp up is caused in my opinion by more and more GPUs mining.

Regards.