Stamping Before Current Visa Expiry..

Hi… I am on H1B Visa with stamping which is valid till 30 Sept 2015.
I got extension and the I797 has valid date from 29 Sept 2015 to 29 Sept 2018.

I am planning to visit India for stamping by June end. So do I get new Visa stamping with Start date 29 Sept 2015 ?

Can I travel to US after 3 weeks of vacation?

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