Always use sockets over cores inside a VM unless you have specific requirements (like licensing). It may simply come down to you needing more vCPUs configured, but a tool like vROps would be able to tell you more.
what is the vendor recommended for setup?
use CPU and Memory reservation for these 6 servers that is how you can avoid over commit.
So if I should give all the servers 2 Sockets and 8 Cores, this would not be a problem?
1. Use ESXTOP and See the virtual machine performance. Share the Matrix here if you can. Also you can use vRops if it is configured in your environment.
2. Specifically look for %idle %ready and %costop
3. Do not increase vCPU in case %idle is high on that VM.
4. Enable CPU and Memory reservation.
5. Set Power Managment setting to "High Performance" at ESXi as well as On Hardware BIOS Setting.