RCA outputs to COAX input - is there only a splitter or is a extra device needed

The Vault has optical toslink output use that. 

Audio

Analog RCA stereo jack, TOSLINK digital optical output

Thnx but I have to wonder how the quality of that signal will be ?

You can't use analog out from the vault to digital in on a dac. There is no splitter to do that. These dacs will work or any other with analog in.

https://benchmarkmedia.com/collections/all-products/products/benchmark-dac3-l-digital-to-analog-audi...

https://nadelectronics.com/product/c-658-bluos-streaming-dac/

Get a quality build multi-strand real glass fibre toslink cable, and bypass the cheap crap plastic ones. The audio quality step up is big and top-shelf. 

I have bought two brands that I use: WIRE-WORLD TOSLINK SUPER NOVA 7 and DH Labs GLASS MASTER TOSLINk

Both are good, but in my bake-offs, the more expensive WIREWORLD was a step-up
https://6moons.com/audioreviews2/wireworld/2.html

The only other way is use an ADC then any DAC would work.

I would try the mini plug out with the optical cable first and see how it works.

Thank you both for your help here. So I am learning and see that I do have some choices to make and comparison shopping to do. Purchasing a  new DAC that has streaming capabilities would mean selling the Vault which frankly doesn’t phase me except going through the pain of listing selling shipping etc. or as you suggest Dj getting an analog to digital converter and getting one of  the DACs I mentioned in my post ( would this be cost effective compared to going for the high end glass Toslink  which eliminates the need for the ad converter? I’ll be reading up on the links you provided. Thanks again.