Thanks for you all give me so many information and opinions,
so may I conclude as below ?
1. VLS is not a limitation nor a speed pilot always have to be at or above, but only a lowest selectable speed for autothrust.
2. Pilot can fly a speed below VLS momentarily with a good reason. It is a common sense, or check/test standard in most company, not words in FCOM or FCTM.
3. VLS is a speed with sufficient margin to stall or activate Airbus protection.
4. In a check/test, normally speed tolerance is +10kts/-5kts.
That's what and why I answered when he asked how do I feel :
I confess I'm not flying good, but I don't violate anything, and I'm still in a positive climb, which is the most important while in such low altitude with a plane get into the runway. I don't understand why I should wait until the power comes up just to maintain a speed at or above VLS, or the manage speed target. It is a high bypass ratio Engine.