Since you already have a great DAC, you can beat that $600/1,200 price on a server/streamer by a country mile, but it's a little bit of DIY work to put that together.
Many of us use the Raspberry Pi as both a server, and a streamer. I break that job into those two parts, whereas something like a Vault is both a server and a streamer all in one.
It's not super difficult, but it's also not an off-the-shelf plug and play kind of thing either.
For the server side, I use a Raspberry Pi with a USB hard drive attached that houses the actual tracks, your Bluesound backup could be that same thing and no there shouldn't be any file incompatibility issues, I believe those Vault rips are FLAC, no?
Anyway, take a look at the the
JRiver Id, it's an example of what can be done very inexpensively and the sonic result is nothing shy of outstanding. I use the Id Pi version, which runs on Raspberry Pi hardware instead of a NUC. Thats the server with the USB hard drive connected to it.
I did a thread on the Id Pi you can
look at here, mine has been through 3 different versions now, as I like to upgrade when newer better versions of the RPi board come out, not because there was any problem with the previous version, but because it's flat out cheap to do so, why not then? I still have all 3 iterations and could plug any of them in and be up and running with it in no time. That thread isn't long, just 14 total posts.
A higher end version of the server side piece would be something like the
Small Green Computer sonicTransporter i5 Gen. 3.
We also have a few different threads on use of that very same RPi board in a streamer/endpoint. That unit connects to the DAC via USB or SPDIF, and the separate server above streams to it over your network, using either Ethernet or WiFi.
But if off-the-shelf plug n' play is more your speed, it costs a lot more, but there are certainly alternatives to Bluesound for consideration. Anything from a Cambridge Audio, Denon/Marantz HEOS, Yamaha, etc... all compete with Bluesound, and the next generation of Sonos does too as it now supports hi-rez unlike the original Sonos products which were restricted to 16/48.
Higher end solutions include Lumin, Innuos, Sonore, Aurender, and various others. But for 1,200 clams I think something like the
Innuos Zen Mini Mk 3 smokes a Bluesound Vault, but that's just my opinion. Better design, and better sound too.
About $1,400 would get you a
server and a streamer in the high-end realm, the sonicTransporter and microRendu will blow the pants off the Vault, though not exactly cheap like the DIY Raspberry Pi stuff is.