A ready to use 8051 dev environment for VS Code
I recommand using the ASEM-51 assembler since (I think) it's the most advanced one out there.
8051-assembly has all the features you need. Just head over to the project page and follow the installation instructions.
This environment uses a Makefile for the build process, so it would be a good idea to install this program as well.
If you're on Arch Linux you can just install these packages: asem51-bin (aur) and make.
On Debian-based distros you need to install the deb file manually from the ASEM-51 project page. make should be in your package manager.
After cloning the repo, open it up in your VS Code installation. Your main code goes into main.a51.
To start the build process press CTRL + SHIFT + B or type Run build task in the Command Palette.