UK passport renewal and visa in old passport still valid?

Hello, my name is Robert Cottis,

I have a work permit in Vietnam, It expires on 25.9.2015.

Currently I am home in the UK and will return to Vietnam on 5.9.2015. (my company were planning on renewing my work permit for a second year when i arrive in vietnam)

Recently, I have renewed my passport as it will expire in March 2016.

Now I have 2 passports, my old passport (which has been clipped, and has the current valid work permit) and my new passport.



I am wondering if i can enter Vietnam using both passports, as my old passport contains my current visa? (the old passport and visa has not expired)

or

If I will need to apply for a new business visa here in the UK using my new passport.

Your reply is much appreciated,
Thank you