How to enable VSYNC in D3D windowed app?

Question

So, Im using D3D in a windowed application.

I inited D3D with the following parameters:

windowed: true;
backbufferformat: D3DFMT_X8R8G8B8;
presentinterval: D3DPRESENT_INTERVAL_ONE;
swapeffect: DISCARD

Each time OnPaint is called, I render the image to the backbuffer and present it to front.

As far as I know (and so does MSDN say), once I set D3DPRESENT_INTERVAL_ONE, vsync will work.

But in this case, the image is teared when dragging horizontally.

(It seems there's a line across the image, image below the line shows on the monitor and the above part follows.)

Some sites say D3DPRESENT_INTERVAL_ONE will not work in windowed applications.

How can I enable vsync anyway?

p.s. I finally found D3D vsync is enabled, while some window settings are not right that perhaps the window itself is not sync ed. though, I haven't peek the settings out.