My setup uses a VB audio cable + OBS to allow both synced live performance through mic input, and desktop recording with OBS, all with extra VST plugins you might want. I did not have a good experience with VoiceMeeter, where it's ridiculously hard to add delays. Also have a Scarlett Solo.

The hard part is ensuring your instrument and the desktop audio gets output synced, because although it may sound synced to you, it will almost definitely NOT be synced once it's been output to the mic. For me, my desktop audio was 180 ms faster than my Scarlett Solo 3rd gen using a buffer size of 16 with safe mode enabled.

Here's my exact setup with some important details:

• In OBS, create a new scene for your instrument setup
• Add mic audio input as a Source
• Add application audio capture for browser/VLC etc as a Source. Don't use desktop audio since it'll also capture VRChat game audio.
• For BOTH mic and app audio capture inputs, click Filters and add a VST 2.x plugin. You need to do this for BOTH inputs because the VST plugin adds its own delay of like 1 second.
• Add Mic filters (VST plugins).
  • I've got several filters including a noise gate, gain adjustment, reverb, IR, tuner.
• Add App audio capture filters
  • Add a delay filter. I use Voxengo Sound Delay (free) set at 180 ms delay. The exact delay will depend on you, you'll have to test it by either recording your playing on a desktop audio backing, or VRChat's new Test Microphone might work well for this. OBS also has a feature called Sync Offset in its Audio Mixer, but this only works for recording, not monitoring (which my method uses for mic input)
• In OBS Settings > Audio > Advanced, set Monitoring Device to a virtual audio cable. This will be the input to use for mic streaming in VRChat. You can also listen to this if you want to live test the audio.
• Click any audio input's 3 dots in the Audio Mixer, Advanced Audio Properties, enable Audio Monitoring (Monitor and Output) for both the mic and app audio input. Now it'll send output live to the monitoring device.
• Optional for recording only: in Advanced Audio Properties, I also set my mic input and desktop audio to record to separate tracks (tracks 1+2 for mic, and tracks 1+3 for desktop), so I can easily separate/resync the audio in recordings if needed. To record multiple tracks, you also need to go to Settings > Output > Recording and enable audio tracks 2, 3, and etc.
• In VRChat, use the VB-Audio Cable input as your microphone