I’m no expert on the subject, but I’m not sure the virtual cores of a VPS are comparable at all to physical cores of a CPU. I guess that to have a broad approximation, you could look at the amount of RAM offered, and assume they run these VPS on a 256 Mo computer. Contabo with their “10 (virtual) cores” offers 60 Mo of RAM. They probably run 4 of these on something approx. equivalent to the Hetzner dedicated server you mention (a server with 256 Mo). So, I would assume the “10 (virtual) cores” are approx a quarter of the performance of the “6 (physical) cores”. Let’s say half tops if they run 2 of them on a 128 Mo computer (which may have a better or slower CPU than the Hetzner machine, who knows).
(You can really only compare “virtual cores” between the different VPS offers of the same provider. I’m not even sure it makes sense for comparisons of VPSes of different providers).
If you want to know for sure, you have to subscribe for 1 month and run a benchmark. Or convince them to run it for you, or offer you a free trial for a short period of time. If it’s really performance you’re after, that’s the real only way to know for VPSes. Or go with a dedicated server where you have more accurate informations.
EDIT: VPS providers like Contabo usually use KVM to do virtualization, and after a search, it indeed seems you can show more “virtual cores” than you actually have physical cores. Example:
Yes, you can configure and run more virtual CPUs than physical cores available. This is an idle Windows Server 2008 (x86, German) KVM guest showing 10 CPUs on a 4-core single-CPU Intel X3210 machine
(source: https://serverfault.com/questions/435231/can-kvm-cpu-assignment-count-differ-from-physical-hosts-cpu-count )
It would make sense they do this seen how much RAM they offer compared to the number of “cores”.