Enabling Lighting & ARB vertex program

I have a stange problem that occurs on NVidia board (on a GeForce FX5200 - driver 53.03, on a GeForce 4Ti - driver 53.03) and on a ATI 9700 Pro:

I made a big scene with the same vertex program to draw each vertex buffer object. The vertex program just computes the lighting with one directional light.
But if I let the lighting enabled (glEnabled(GL_LIGHTING) ) my application frame rate drop down whereas if I turn off the standard lighting (glDisable(GL_LIGHTING)) the frame rate is the same as if I don’t use the vertex program !

Why am I obliged to turn off the standard lighting ?!
Do I do something wrong ?
Have you ever heard about such a problem ?

Thanks for all

Note: I use the CG toolkit to produce the program.

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