ShingenPizza
With some resolutions set in the Steam's launch options, when starting in fullscreen, VRChat will start rendering in that resolution instead of the screen's one, making everything blurry.
Tested with a regular 1920x1080 screen
Steps to reproduce:
  1. In Steam's VRChat properties set launch options: -screen-width 640 -screen-height 360
===== If you haven't used fullscreen mode at your previous session:
|| 2. Run VRChat
|| 3. VRChat window will start with its content rendered at given resolution
|| 4. press Alt+Enter to go fullscreen, the content will be rendered at your screen's resolution instead
|| 5. Close VRC (while still being in the fullscreen mode!)
=====
  1. Run VRChat
  2. VRChat will start in fullscreen mode, but rendering at the 640x360 (or similarly low) resolution, with everything being blurry <- this is the reported problem
  3. Press Alt+Enter, you will quit fullscreen, and see a small window at a given resolution
  4. Press Alt+Enter again, you will enter fullscreen mode again, but this time correctly rendering at your screen's resolution
  5. Press Alt+Enter, you will quit fullscreen, but this time with the window being maximized, which is a smaller one, but IMO also a bug
Funnily enough, if you set the launch option resolution to something like 128x72, at #3 the VRC window will be of (at least approximately) given resolution, while in #7 it will override that tiny resolution with the screen's one - but for whatever reason it doesn't do that for a 'medium' resolution of 640x360.
Attached visualization of steps 7-9 (starring: Photographotter).