Can I Do Veneer Preparation Directly After Crown Lengthening and Take Impression for my Patient in Same Session??

If you are going to do flap surgery with any osseous (bone) reshaping you would wait 4-6 weeks.  If just gingival recontouring with scalpel, bur, diamond or electrosurge you should wait until healing, which is as long as it takes.  If you are speaking about only doing laser gingival recontouring, often times the impression can be done the same day because laser is significantly less invasive and gets quicker healing than the other formentioned procedures.  I'm not trying to be disrespectful, but are you sure that you are a dentist and licensed?  Have you had any formal training doing veneers and crown lengthening?  For dental questions regarding how a dentist does dentistry you might be better asking your questions on a dental forum.

if you are doing only soft contouring with any method, especially a diode laser, then you can take the impression directly after the tissue has been reshaped. If it osseous recontouring that is also involved then you will need to wait if it is done with an open flap procedure. However if you are using a waterlase and doing a closed procedure, you can take the impression the same day.

I have successfully taken final impressions following conservative gingival recontouring but would advise against it following crown lengthening, as it is hard to predict the final gingival position.  I would recommend allowing at least 4 to 6 weeks of healing.