This happens because you have mount helpers which mount the storage devices with the identity of the user holding the current session on your machine. If you have a GUI and log in as pi, that's the user who will own the mounted devices.
Changing permissions on /media/pi will not achieve anything as the mount helpers will either reset the permissions or ...
Here's the relevant part of the official docs.
Min and max:
GPU memory in megabytes, sets the memory split between the CPU and GPU; the CPU gets the remaining memory. The minimum value is 16; the technical maximum value 944 [for the RPi 4]. The default value is 64, values above 512 will not provide increased performance and should not be used.
You shouldn't need to set it beyond the default value in the plain Raspbian /boot/config.txt. Max value is 128M.
Setting it beyond 128M can leave your Raspberry Pi 4B unbootable.
GPU memory on a RPi4B isn't used in the same way as on earlier Raspberries.
I also use VLC on the latest Buster and RPi4. I remember getting some screen blinking when I used a crappy HDMI cable which I have since replaced. One easy test you can do is changing the screen resolution to a lower one, e.g. 1280x720. You can do this e.g. by editing /boot/config.txt, running raspi-config (those two require a reboot), by going to ...