I have an old Windows laptop. I need to figure out how to do dual boot with Linux

For this I would recommend:

1. Install Windows first
2. In Windows, partition the disk drive to how much storage you want. So if you have a 1TB, then maybe do 500GB for Windows and 500GB for Linux? Leave the new partition as unformatted/unallocated
3. Boot up your linux installer and select the unformatted/unallocated partition for Linux to install to. Don't erase whole disk. But let Linux setup all of it's own formatting and partitions on the empty space

Now why do it this way? Because Windows does NOT like the boot manager being replaced and does NOT like disk space go "missing" unless it allocates it itself. If you install Windows first it'll setup the boot manager for Windows and then when you install Linux grub will get installed and that can manage Windows pretty well.

And if you let Windows partition off the blank space for Linux then Windows knows that that empty partition isn't owned by Windows anymore and it won't freak out seeing the space go missing when Linux takes it over.

This article covers most: https://linuxblog.io/dual-boot-linux-windows-install-guide/

If you have two individual disk drives then I would do the same thing, install Windows on one of the drives, boot into Windows, and make sure the second drive shows up in disk utility, but it isn't formatted for use in Windows, just unallocated/blank. Then when you install Linux you just tell it to install onto the second drive.

and get my vpn sorted (again) so he can use VMs on my Proxmox box

I would 100% recommend Tailscale for this. You can install Tailscale on the Proxmox host and then have your nephew have his own Tailscale account where you can give him access to only the Proxmox box.

I do this with my Proxmox boxes so I can remotely manage them wherever I am. When you first install Tailscale on Proxmox it may require a reboot, so I would recommend being nearby the server so you can login physically if needed, but after it has been smooth sailing for me. Been using it like this for a year or two now.

Of course just a suggestion.