I really liked the BluOS app. It’s a very versatile app with plethora of options to stream. That’s one thing I missed with current App. By all means, keep the Vault and get a better DAC. As others have reported, you can also improve Vault’s performance by upgrading its power supply plus good quality SPDIF cable.
As far as picking a DAC, search online and keep an eye on used market here and on USAM. I am confident you can find something within your budget and to your liking. It’s a bit of work but well worth it.
Good luck!