What does 1 vcpu mean precisely/exactly? When we are talking about "1 vcpu" in different cloud solutions: 

Q: What exactly does it mean? 

Ex.: if I will use a 10 vcpu machine for the same cpu intensive process (which only uses CPU) then I would see the same amount of CPU load on them? Would they finish at the same time regarding the same only-CPU-usage task? 

Or the definition of vcpu could differ in the different cloud solutions?
vcpu = Virtual CPU, that's just the number of core of the instance. Nothing more, nothing less.

For the rest of your questions they're unrelated, this depends on the underlying hypervisor's physical processors, there's no guarantee they use the same type with same frequency.