I think this will answer your question.
There is no answer. It's just an explanation of VBlankSync flag and I already know it much deeper than it explained in your link.
Using VBlankSync flag in Dircect3D SwapChain::Present function reduces CPU load if it called much faster than physical VBlank event occurs.
For example, if your render time is 1/10 of VBlank period, using VBlankSync will reduce CPU usage for about 10x times. So, one core CPU usage will be for about 10% (or little higher) instead of 100%.
If your render time is 1/1000 of VBlank period (it's the case I'm talking about), using VBlankSync will reduce CPU usage for about 1000 times. So, one core CPU usage will be for about 0,1% (or little higher) instead of 100%.
But the problem is that with SFML it is not reduced and still remains 200% and it doesn't matter if you're using VBlankSync flag or not (in a counterweight to uisng Direct3D, where it is really reduced with VBlankSync enabled).
Actually there are two ways to implement VBlankSync:
1) Using VBlankSync flag, so the call to SwapChain::Present will be blocked until VBlank event will occurs. It's usual way which is used in most of graphics software, especially it must have for windowed mode applications.
2) Using "Spinlock" loop with direct scanning of VBlankSync flag. It's may be used internally inside video driver if an application works in fullscreen mode. Such technique allows to minimize latency time between physical VBlank event and software response (command to swap the buffers).
Spinlock means that CPU executes special opcode "pause". This opcode notifies CPU that he can relax for one cpu cycle and use this time to optimize it's cache, but CPU should NOT switch thread context. So, such instruction prevents CPU to switch thread execution and locks execution in the current thread.
It increases CPU usage, because CPU core cannot do useful work in another thread and continues to execute idle loop which goal is to check physical flag.
Spinlock approach is appropriate technique for fullscreen graphics mode, when application works in exclusive mode.
But it's not appropriate for the case when application works in windowed mode and shares videocard with another applications.
SFML behavior shows that it probably uses spinlock wait in windowed mode (CPU usage testifies it). And it's the problem. Such CPU usage should not happens in windowed mode with VBlankSync enabled.