VirtualBox seems to have a tiny limit on VRAM, luckily there’s a way around it!

Open your terminal and run the following, with your VM name in the quotation marks.

vboxmanage modifyvm "Arch Linux VM" --vram 256

Now open VirtualBox and you will see the amount of video memory has been increased to 256Mb, double the default limit. You can go as high as your hardware allows :)