Are vCPU the same as 1 Socket, or a single Core?

Hyper-V presents one socket with multiple cores.

You can verify this with a tool such as CPU-Z. Here's a screenshot I took of a 2-vCPU Hyper-V guest I have:

For what it's worth, VMWare ESXi permits you to choose your mix of sockets/cores vCPUs.

I am not a lawyer, so stop reading now

Installing SQL Express on a machine that violates the license simply means (as far as I can tell) that the SQL instance will not utilise the extra power of the server. For example, it's limited to x gigs of ram, but if you install it in a server with 256Gb of RAM, it will just neglect to use the rest.