Different protocols and hardware used for different connections have different bandwidth limitations. Sometimes the limits are self-imposed by DAC designers who don’t have confidence in the tolerances of a connection above a certain bandwidth.
This post by Jussi Lasko is the best description I have read of what happens in a typical delta-sigma chip DAC, such as the Vega.
There can be advantages to bypassing upsampling within a DAC by doing it in a more powerful general computer that is isolated from the DAC by Ethernet. That is part of the reason Roon offers the option. See this KB page for more details about Sound Quality and this one for more about Sample Rate Conversion.
If you disable DSP Sample Rate Conversion then Roon will never up/downsample. If you enable “For Compatability Only” then Roon will only up/downsample to enable an otherwise incompatible file (sampling rate lower or higher than your DAC accepts) to be played by your DAC. Roon will know what your DAC can accept over various inputs if the DAC is Roon Tested (which the Vega is).
So if you prefer to upsample in the Vega, which is a perfectly valid choice that Auralic might reccomend, then you can still use Roon up/downsampling for compatability purposes only. The Signal Chain will tell you if this is happening. In the case of the Vega it would happen if you played a DSD file into the AES input for example. Roon would convert the file to PCM 192 kHz so it was compatible with the selected DAC input. If you connected the Aries to the Vega by the USB input, then Roon would pass through or downsample the file to the DSD capability of the Vega USB input.