I have started vps mining just to know how it works.
but i want to know

that whenever i close putty it stops mining.
do anyone know which command i should run to let vps mine 24 hours a day.
i am using debian 7.0 x64 in my vps.
please help.
This has been unprofitable for some time now.
LTC mining was profitable a few months ago when using cloud computing, but difficulty increases and price decreases have caused it to become unprofitable.
My understanding is that the most one instance can mine is 2-3 MH/s.
Per
http://www.bitcoinx.com/profit/ 200 GH/s will mine .0086 BTC per 24 hour period.
Based on the above numbers you would need to have ~66 thousand instances for 24 hours to earn .0086 BTC which comes out to $5
You are wrong. AFAIK, most instance can't get pass 500kh/s. LTC was never profitable since hosting providers will shut your VPS down after using 50% of the CPU for too long.